diff --git a/backend/app/models/post.rb b/backend/app/models/post.rb
index 24af551..1bd0723 100644
--- a/backend/app/models/post.rb
+++ b/backend/app/models/post.rb
@@ -9,12 +9,7 @@ class Post < ApplicationRecord
   has_many :post_tags_with_discarded, -> { with_discarded }, class_name: 'PostTag'
   has_many :tags, through: :active_post_tags
   has_many :user_post_views, dependent: :destroy
-  has_many :post_similarities_as_post,
-           class_name: 'PostSimilarity',
-           foreign_key: :post_id
-  has_many :post_similarities_as_target_post,
-           class_name: 'PostSimilarity',
-           foreign_key: :target_post_id
+  has_many :post_similarities
   has_one_attached :thumbnail
 
   before_validation :normalise_url
@@ -34,18 +29,12 @@ class Post < ApplicationRecord
   end
 
   def related(limit: nil)
-    ids_with_cos =
-        post_similarities_as_post.select(:target_post_id, :cos)
-                                 .map { |ps| [ps.target_post_id, ps.cos] } +
-        post_similarities_as_target_post.select(:post_id, :cos)
-                                        .map { |ps| [ps.post_id, ps.cos] }
+    ids = post_similarities.select(:target_post_id).order(cos: :desc)
+    ids = ids.limit(limit) if limit
+    ids = ids.pluck(:target_post_id)
+    return [] if ids.empty?
 
-    sorted = ids_with_cos.sort_by { |_, cos| -cos }
-
-    ids = sorted.map(&:first)
-    ids = ids.first(limit) if limit
-
-    Post.where(id: ids).index_by(&:id).values_at(*ids)
+    Post.where(id: ids).order(Arel.sql("FIELD(id, #{ ids.join(',') })"))
   end
 
   def resized_thumbnail!
diff --git a/backend/app/models/post_similarity.rb b/backend/app/models/post_similarity.rb
index 0f86550..c549b5e 100644
--- a/backend/app/models/post_similarity.rb
+++ b/backend/app/models/post_similarity.rb
@@ -1,6 +1,6 @@
 class PostSimilarity < ApplicationRecord
   self.primary_key = :post_id, :target_post_id
 
-  belongs_to :post, class_name: 'Post', foreign_key: 'post_id'
-  belongs_to :target_post, class_name: 'Post', foreign_key: 'target_post_id'
+  belongs_to :post
+  belongs_to :target_post, class_name: 'Post'
 end
diff --git a/backend/app/models/tag_similarity.rb b/backend/app/models/tag_similarity.rb
index 7a6d252..dd66c1e 100644
--- a/backend/app/models/tag_similarity.rb
+++ b/backend/app/models/tag_similarity.rb
@@ -1,6 +1,6 @@
 class TagSimilarity < ApplicationRecord
   self.primary_key = :tag_id, :target_tag_id
 
-  belongs_to :tag, class_name: 'Tag', foreign_key: 'tag_id'
-  belongs_to :target_tag, class_name: 'Tag', foreign_key: 'target_tag_id'
+  belongs_to :tag
+  belongs_to :target_tag, class_name: 'Tag'
 end
diff --git a/backend/app/services/similarity/calc.rb b/backend/app/services/similarity/calc.rb
new file mode 100644
index 0000000..40d717d
--- /dev/null
+++ b/backend/app/services/similarity/calc.rb
@@ -0,0 +1,106 @@
+module Similarity
+  class Calc
+    def self.call model, tgt
+      similarity_model = "#{ model.name }Similarity".constantize
+
+      # 最大保存件数
+      n = 20
+
+      similarity_model.delete_all
+
+      posts = model.includes(tgt).select(:id).to_a
+
+      tag_ids = { }
+      tag_cnts = { }
+
+      posts.each do |p|
+        arr = p.public_send(tgt).map(&:id).sort
+        tag_ids[p.id] = arr
+        tag_cnts[p.id] = arr.size
+      end
+
+      intersection_size = -> a, b do
+        i = 0
+        j = 0
+        cnt = 0
+        while i < a.size && j < b.size
+          a_i = a[i]
+          b_j = b[j]
+          if a_i == b_j
+            cnt += 1
+            i += 1
+            j += 1
+          elsif a_i < b_j
+            i += 1
+          else
+            j += 1
+          end
+        end
+        cnt
+      end
+
+      push_topk = -> list, cos, target_id do
+        return if list.size >= n && cos <= list[-1][0]
+
+        idx = nil
+        list.each_with_index do |(c, tid), i|
+          if tid == target_id
+            idx = i
+            break
+          end
+        end
+        if idx
+          return if cos <= list[idx][0]
+          list.delete_at(idx)
+        end
+
+        insert_at = list.size
+        list.each_with_index do |(c, _), i|
+          if cos > c
+            insert_at = i
+            break
+          end
+        end
+        list.insert(insert_at, [cos, target_id])
+        list.pop if list.size > n
+      end
+
+      top = Hash.new { |h, key| h[key] = [] }
+
+      ids = posts.map(&:id)
+      ids.each_with_index do |post_id, i|
+        a = tag_ids[post_id]
+        a_cnt = tag_cnts[post_id]
+
+        ((i + 1)...ids.size).each do |j|
+          target_id = ids[j]
+          b = tag_ids[target_id]
+          b_cnt = tag_cnts[target_id]
+
+          norm = Math.sqrt(a_cnt * b_cnt)
+          cos = norm.zero? ? 0.0 : intersection_size.(a, b).fdiv(norm)
+
+          push_topk.(top[post_id], cos, target_id)
+          push_topk.(top[target_id], cos, post_id)
+        end
+      end
+
+      buf = []
+      flush = -> do
+        return if buf.empty?
+        similarity_model.insert_all!(buf)
+        buf.clear
+      end
+
+      top.each do |post_id, list|
+        list.each do |cos, target_post_id|
+          buf << { "#{ model.name.underscore }_id".to_sym => post_id,
+                   "target_#{ model.name.underscore }_id".to_sym => target_post_id,
+                   cos: }
+          flush.call if buf.size >= 1_000
+        end
+      end
+      flush.call
+    end
+  end
+end
diff --git a/backend/db/migrate/20260121213900_add_index_to_post_similarities.rb b/backend/db/migrate/20260121213900_add_index_to_post_similarities.rb
new file mode 100644
index 0000000..52a6ee1
--- /dev/null
+++ b/backend/db/migrate/20260121213900_add_index_to_post_similarities.rb
@@ -0,0 +1,9 @@
+class AddIndexToPostSimilarities < ActiveRecord::Migration[8.0]
+  def change
+    remove_index :post_similarities, name: 'index_post_similarities_on_post_id'
+
+    add_index :post_similarities, [:post_id, :cos],
+              order: { cos: :desc },
+              name: 'index_post_similarities_on_post_id_and_cos'
+  end
+end
diff --git a/backend/db/migrate/20260121225600_add_index_to_tag_similarities.rb b/backend/db/migrate/20260121225600_add_index_to_tag_similarities.rb
new file mode 100644
index 0000000..c820f15
--- /dev/null
+++ b/backend/db/migrate/20260121225600_add_index_to_tag_similarities.rb
@@ -0,0 +1,9 @@
+class AddIndexToTagSimilarities < ActiveRecord::Migration[8.0]
+  def change
+    remove_index :tag_similarities, name: 'index_tag_similarities_on_tag_id'
+
+    add_index :tag_similarities, [:tag_id, :cos],
+              order: { cos: :desc },
+              name: 'index_tag_similarities_on_tag_id_and_cos'
+  end
+end
diff --git a/backend/db/schema.rb b/backend/db/schema.rb
index ee14689..c6fc0ed 100644
--- a/backend/db/schema.rb
+++ b/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_01_21_024800) do
+ActiveRecord::Schema[8.0].define(version: 2026_01_21_225600) 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
@@ -59,7 +59,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_01_21_024800) do
     t.bigint "post_id", null: false
     t.bigint "target_post_id", null: false
     t.float "cos", null: false
-    t.index ["post_id"], name: "index_post_similarities_on_post_id"
+    t.index ["post_id", "cos"], name: "index_post_similarities_on_post_id_and_cos", order: { cos: :desc }
     t.index ["target_post_id"], name: "index_post_similarities_on_target_post_id"
   end
 
@@ -130,7 +130,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_01_21_024800) do
     t.bigint "tag_id", null: false
     t.bigint "target_tag_id", null: false
     t.float "cos", null: false
-    t.index ["tag_id"], name: "index_tag_similarities_on_tag_id"
+    t.index ["tag_id", "cos"], name: "index_tag_similarities_on_tag_id_and_cos", order: { cos: :desc }
     t.index ["target_tag_id"], name: "index_tag_similarities_on_target_tag_id"
   end
 
diff --git a/backend/lib/tasks/calc_post_similarities.rake b/backend/lib/tasks/calc_post_similarities.rake
index 97e4fd2..925770f 100644
--- a/backend/lib/tasks/calc_post_similarities.rake
+++ b/backend/lib/tasks/calc_post_similarities.rake
@@ -1,28 +1,6 @@
 namespace :post_similarity do
   desc '関聯投稿テーブル作成'
   task calc: :environment do
-    dot = -> a, b { (a.keys & b.keys).sum { |k| a[k] * b[k] } }
-    norm = -> v { Math.sqrt(v.values.sum { |e| e * e }) }
-    cos = -> a, b do
-      na = norm.(a)
-      nb = norm.(b)
-      if na.zero? || nb.zero?
-        0.0
-      else
-        dot.(a, b) / na / nb
-      end
-    end
-
-    posts = Post.includes(:tags).to_a
-    posts.each_with_index do |post, i|
-      existence_of_tags = post.tags.index_with(1)
-      ((i + 1)...posts.size).each do |j|
-        target_post = posts[j]
-        existence_of_target_tags = target_post.tags.index_with(1)
-        PostSimilarity.find_or_initialize_by(post:, target_post:).tap { |ps|
-          ps.cos = cos.(existence_of_tags, existence_of_target_tags)
-        }.save!
-      end
-    end
+    Similarity::Calc.call(Post, :tags)
   end
 end
diff --git a/backend/lib/tasks/calc_tag_similarities.rake b/backend/lib/tasks/calc_tag_similarities.rake
index 9e16977..0fc718a 100644
--- a/backend/lib/tasks/calc_tag_similarities.rake
+++ b/backend/lib/tasks/calc_tag_similarities.rake
@@ -1,28 +1,6 @@
 namespace :tag_similarity do
   desc '関聯タグ・テーブル作成'
   task calc: :environment do
-    dot = -> a, b { (a.keys & b.keys).sum { |k| a[k] * b[k] } }
-    norm = -> v { Math.sqrt(v.values.sum { |e| e * e }) }
-    cos = -> a, b do
-      na = norm.(a)
-      nb = norm.(b)
-      if na.zero? || nb.zero?
-        0.0
-      else
-        dot.(a, b) / na / nb
-      end
-    end
-
-    tags = Tag.includes(:posts).to_a
-    tags.each_with_index do |tag, i|
-      existence_of_posts = tag.posts.index_with(1)
-      ((i + 1)...tags.size).each do |j|
-        target_tag = tags[j]
-        existence_of_target_posts = target_tag.posts.index_with(1)
-        TagSimilarity.find_or_initialize_by(tag:, target_tag:).tap { |ts|
-          ts.cos = cos.(existence_of_posts, existence_of_target_posts)
-        }.save!
-      end
-    end
+    Similarity::Calc.call(Tag, :posts)
   end
 end