backend/app/controllers/theatres_controller.rb

@@ -0,0 +1,52 @@
+class TheatresController < ApplicationController
+  def show
+    theatre = Theatre.find_by(id: params[:id])
+    return head :not_found unless theatre
+
+    render json: TheatreRepr.base(theatre)
+  end
+
+  def watching
+    return head :unauthorized unless current_user
+
+    theatre = Theatre.find_by(id: params[:id])
+    return head :not_found unless theatre
+
+    host_flg = false
+    post_id = nil
+    post_started_at = nil
+
+    theatre.with_lock do
+      TheatreWatchingUser.find_or_initialize_by(theatre:, user: current_user).tap {
+        _1.expires_at = 30.seconds.from_now
+      }.save!
+
+      if (!(theatre.host_user_id?) ||
+          !(theatre.watching_users.exists?(id: theatre.host_user_id)))
+        theatre.update!(host_user_id: current_user.id)
+      end
+
+      host_flg = theatre.host_user_id == current_user.id
+      post_id = theatre.current_post_id
+      post_started_at = theatre.current_post_started_at
+    end
+
+    render json: { host_flg:, post_id:, post_started_at: }
+  end
+
+  def next_post
+    return head :unauthorized unless current_user
+
+    theatre = Theatre.find_by(id: params[:id])
+    return head :not_found unless theatre
+    return head :forbidden if theatre.host_user != current_user
+
+    post = Post.where("url LIKE '%nicovideo.jp%'")
+           .or(Post.where("url LIKE '%youtube.com%'"))
+           .order('RAND()')
+           .first
+    theatre.update!(current_post: post, current_post_started_at: Time.current)
+
+    head :no_content
+  end
+end

backend/app/models/theatre.rb

@@ -0,0 +1,13 @@
+class Theatre < ApplicationRecord
+  include MyDiscard
+
+  has_many :comments, class_name: 'TheatreComment'
+  has_many :theatre_watching_users, dependent: :delete_all
+  has_many :active_theatre_watching_users, -> { active },
+           class_name: 'TheatreWatchingUser', inverse_of: :theatre
+  has_many :watching_users, through: :active_theatre_watching_users, source: :user
+
+  belongs_to :host_user, class_name: 'User', optional: true
+  belongs_to :current_post, class_name: 'Post', optional: true
+  belongs_to :created_by_user, class_name: 'User'
+end

backend/app/models/theatre_comment.rb

@@ -0,0 +1,8 @@
+class TheatreComment < ApplicationRecord
+  include MyDiscard
+
+  self.primary_key = :theatre_id, :no
+
+  belongs_to :theatre
+end
+

backend/app/models/theatre_watching_user.rb

@@ -0,0 +1,13 @@
+class TheatreWatchingUser < ApplicationRecord
+  self.primary_key = :theatre_id, :user_id
+
+  belongs_to :theatre
+  belongs_to :user
+
+  scope :active, -> { where('expires_at >= ?', Time.current) }
+  scope :expired, -> { where('expires_at < ?', Time.current) }
+
+  def active? = expires_at >= Time.current
+
+  def refresh! = update!(expires_at: 30.seconds.from_now)
+end

backend/app/representations/theatre_repr.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+
+module TheatreRepr
+  BASE = { only: [:id, :name, :opens_at, :closes_at, :created_at, :updated_at],
+           include: { created_by_user: { only: [:id, :name] } } }.freeze
+
+  module_function
+
+  def base theatre
+    theatre.as_json(BASE)
+  end
+
+  def many theatre
+    theatre.map { |t| base(t) }
+  end
+end

backend/config/routes.rb

@@ -72,4 +72,11 @@ Rails.application.routes.draw do
       end
     end
   end
+
+  resources :theatres, only: [:show] do
+    member do
+      put :watching
+      patch :next_post
+    end
+  end
 end

backend/db/migrate/20260316082500_create_theatres.rb

@@ -4,9 +4,11 @@ class CreateTheatres < ActiveRecord::Migration[8.0]
       t.string :name
       t.datetime :opens_at, null: false, index: true
       t.datetime :closes_at, index: true
+      t.integer :kind, null: false, index: true
       t.references :current_post, foreign_key: { to_table: :posts }, index: true
       t.datetime :current_post_started_at
       t.integer :next_comment_no, null: false, default: 1
+      t.references :host_user, foreign_key: { to_table: :users }
       t.references :created_by_user, null: false, foreign_key: { to_table: :users }, index: true
       t.timestamps
       t.datetime :discarded_at, index: true

backend/db/migrate/20260317015000_create_theatre_watching_users.rb

@@ -0,0 +1,12 @@
+class CreateTheatreWatchingUsers < ActiveRecord::Migration[8.0]
+  def change
+    create_table :theatre_watching_users, primary_key: [:theatre_id, :user_id] do |t|
+      t.references :theatre, null: false, foreign_key: { to_table: :theatres }
+      t.references :user, null: false, foreign_key: { to_table: :users }, index: true
+      t.datetime :expires_at, null: false, index: true
+      t.timestamps
+
+      t.index [:theatre_id, :expires_at]
+    end
+  end
+end

backend/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2026_03_16_083500) do
+ActiveRecord::Schema[8.0].define(version: 2026_03_17_015000) do
   create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "record_type", null: false
@@ -180,13 +180,27 @@ ActiveRecord::Schema[8.0].define(version: 2026_03_16_083500) do
     t.index ["user_id"], name: "index_theatre_comments_on_user_id"
   end
 
+  create_table "theatre_watching_users", primary_key: ["theatre_id", "user_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "theatre_id", null: false
+    t.bigint "user_id", null: false
+    t.datetime "expires_at", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["expires_at"], name: "index_theatre_watching_users_on_expires_at"
+    t.index ["theatre_id", "expires_at"], name: "index_theatre_watching_users_on_theatre_id_and_expires_at"
+    t.index ["theatre_id"], name: "index_theatre_watching_users_on_theatre_id"
+    t.index ["user_id"], name: "index_theatre_watching_users_on_user_id"
+  end
+
   create_table "theatres", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name"
     t.datetime "opens_at", null: false
     t.datetime "closes_at"
+    t.integer "kind", null: false
     t.bigint "current_post_id"
     t.datetime "current_post_started_at"
     t.integer "next_comment_no", default: 1, null: false
+    t.bigint "host_user_id"
     t.bigint "created_by_user_id", null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
@@ -195,6 +209,8 @@ ActiveRecord::Schema[8.0].define(version: 2026_03_16_083500) do
     t.index ["created_by_user_id"], name: "index_theatres_on_created_by_user_id"
     t.index ["current_post_id"], name: "index_theatres_on_current_post_id"
     t.index ["discarded_at"], name: "index_theatres_on_discarded_at"
+    t.index ["host_user_id"], name: "index_theatres_on_host_user_id"
+    t.index ["kind"], name: "index_theatres_on_kind"
     t.index ["opens_at"], name: "index_theatres_on_opens_at"
   end
 
@@ -295,8 +311,11 @@ ActiveRecord::Schema[8.0].define(version: 2026_03_16_083500) do
   add_foreign_key "tags", "tag_names"
   add_foreign_key "theatre_comments", "theatres"
   add_foreign_key "theatre_comments", "users"
+  add_foreign_key "theatre_watching_users", "theatres"
+  add_foreign_key "theatre_watching_users", "users"
   add_foreign_key "theatres", "posts", column: "current_post_id"
   add_foreign_key "theatres", "users", column: "created_by_user_id"
+  add_foreign_key "theatres", "users", column: "host_user_id"
   add_foreign_key "user_ips", "ip_addresses"
   add_foreign_key "user_ips", "users"
   add_foreign_key "user_post_views", "posts"

frontend/src/App.tsx

@@ -19,6 +19,7 @@ import PostNewPage from '@/pages/posts/PostNewPage'
 import PostSearchPage from '@/pages/posts/PostSearchPage'
 import ServiceUnavailable from '@/pages/ServiceUnavailable'
 import SettingPage from '@/pages/users/SettingPage'
+import TheatreDetailPage from '@/pages/theatres/TheatreDetailPage'
 import WikiDetailPage from '@/pages/wiki/WikiDetailPage'
 import WikiDiffPage from '@/pages/wiki/WikiDiffPage'
 import WikiEditPage from '@/pages/wiki/WikiEditPage'
@@ -47,6 +48,7 @@ const RouteTransitionWrapper = ({ user, setUser }: {
 	    <Route path="/posts/:id" element={<PostDetailRoute user={user}/>}/>
 	    <Route path="/posts/changes" element={<PostHistoryPage/>}/>
 	    <Route path="/tags/nico" element={<NicoTagListPage user={user}/>}/>
+	    <Route path="/theatres/:id" element={<TheatreDetailPage user={user}/>}/>
 	    <Route path="/wiki" element={<WikiSearchPage/>}/>
 	    <Route path="/wiki/:title" element={<WikiDetailPage/>}/>
 	    <Route path="/wiki/new" element={<WikiNewPage user={user}/>}/>

frontend/src/components/TopNav.tsx

@@ -79,6 +79,8 @@ export default (({ user }: Props) => {
 	  { name: '上位タグ', to: '/tags/implications', visible: false },
 	  { name: 'ニコニコ連携', to: '/tags/nico' },
 	  { name: 'ヘルプ', to: '/wiki/ヘルプ:タグ' }] },
+      { name: '上映会', to: '/theatres/1', base: '/theatres', subMenu: [
+	  { name: '一覧', to: '/theatres' }] },
       { name: 'Wiki', to: '/wiki/ヘルプ:ホーム', base: '/wiki', subMenu: [
 	  { name: '検索', to: '/wiki' },
 	  { name: '新規', to: '/wiki/new' },

frontend/src/pages/theatres/TheatreDetailPage.tsx

@@ -1,8 +1,85 @@
+import { useEffect, useState } from 'react'
+import { Helmet } from 'react-helmet-async'
+import { useParams } from 'react-router-dom'
+
+import PostEmbed from '@/components/PostEmbed'
+import MainArea from '@/components/layout/MainArea'
+import { SITE_TITLE } from '@/config'
+import { apiGet, apiPatch, apiPut } from '@/lib/api'
+import { fetchPost } from '@/lib/posts'
+
 import type { FC } from 'react'
 
+import type { Theatre } from '@/types'
+
+type TheatreInfo = {
+    hostFlg:        boolean
+    postId:         number | null
+    postStartedAt:  string | null }
+
+type Props = { user: User }
+
+
+export default (({ user }: Props) => {
+  const { id } = useParams ()
+
+  const [loading, setLoading] = useState (false)
+  const [hostFlg, setHostFlg] = useState (false)
+  const [theatre, setTheatre] = useState<Theatre | null> (null)
+  const [theatreInfo, setTheatreInfo] =
+      useState<TheatreInfo> ({ hostFlg: false, postId: null, postStartedAt: null })
+  const [post, setPost] = useState<Post | null> (null)
+
+  useEffect (() => {
+    if (!(id))
+      return
+
+    void (async () => {
+      setTheatre (await apiGet<Theatre> (`/theatres/${ id }`))
+    }) ()
+
+    const interval = setInterval (async () => {
+      setTheatreInfo (await apiPut<TheatreInfo> (`/theatres/${ id }/watching`))
+    }, 1_000)
+
+    return () => clearInterval (interval)
+  }, [id])
+
+  useEffect (() => {
+    if (!(theatreInfo.hostFlg) || loading)
+      return
+
+    if (theatreInfo.postId == null)
+      {
+	void (async () => {
+	  setLoading (true)
+	  await apiPatch<void> (`/theatres/${ id }/next_post`)
+	  setLoading (false)
+	}) ()
+	return
+      }
+  }, [theatreInfo])
+
+  useEffect (() => {
+    if (theatreInfo.postId == null)
+      return
+
+    void (async () => {
+      setPost (await fetchPost (theatreInfo.postId))
+    }) ()
+  }, [theatreInfo.postId, theatreInfo.postStartedAt])
 
-export default (() => {
   return (
       <MainArea>
+	<Helmet>
+	  {theatre && (
+	      <title>
+		{'上映会場'
+		 + (theatre.name ? `『${ theatre.name }』` : ` #${ theatre.id }`)
+		 + ` | ${ SITE_TITLE }`}
+	      </title>)}
+	</Helmet>
+
+	{post && <PostEmbed post={post}/>}
       </MainArea>)
-}) satisfies FC
+}) satisfies FC<Props>

frontend/src/types.ts

@@ -75,6 +75,15 @@ export type Tag = {
     children?:      Tag[]
     matchedAlias?:  string | null }
 
+export type Theatre = {
+    id:                   number
+    name:                 string | null
+    opensAt:              string
+    closesAt:             string | null
+    createdByUser:        { id: number; name: string }
+    createdAt:            string
+    updatedAt:            string }
+
 export type User = {
     id:               number
     name:             string | null