diff --git a/frontend/src/components/TagDetailSidebar.tsx b/frontend/src/components/TagDetailSidebar.tsx
index 7a54a85..ca4e23c 100644
--- a/frontend/src/components/TagDetailSidebar.tsx
+++ b/frontend/src/components/TagDetailSidebar.tsx
@@ -12,6 +12,7 @@ import TagSearch from '@/components/TagSearch'
 import SectionTitle from '@/components/common/SectionTitle'
 import SubsectionTitle from '@/components/common/SubsectionTitle'
 import SidebarComponent from '@/components/layout/SidebarComponent'
+import { toast } from '@/components/ui/use-toast'
 import { API_BASE_URL } from '@/config'
 import { CATEGORIES } from '@/consts'
 
@@ -55,6 +56,31 @@ const renderTagTree = (
 }
 
 
+const removeEverywhere = (
+    list:   Tag[],
+    tagId:  number,
+): { next: Tag[]; picked?: Tag } => {
+  let picked: Tag | undefined
+
+  const walk = (nodes: Tag[]): Tag[] => (
+      nodes
+      .map (t => {
+	const children = t.children ? walk (t.children) : undefined
+	return children ? { ...t, children } : t
+      })
+      .filter (t => {
+	if (t.id === tagId)
+	  {
+	    picked = picked ?? t
+	    return false
+	  }
+	return true
+      }))
+
+  return { next: walk (list), picked }
+}
+
+
 const addAsChild = (
     list:     Tag[],
     parentId: number,
@@ -76,6 +102,34 @@ const addAsChild = (
 }
 
 
+const attachChildOptimistic = (
+    prev:     TagByCategory,
+    parentId: number,
+    childId:  number,
+): TagByCategory => {
+  const next: TagByCategory = { ...prev }
+
+  let picked: Tag | undefined
+  for (const cat of Object.keys (next) as (keyof typeof next)[])
+    {
+      const r = removeEverywhere (next[cat], childId)
+      next[cat] = r.next
+      picked = picked ?? r.picked
+    }
+  if (!(picked))
+    return prev
+
+  for (const cat of Object.keys (next) as (keyof typeof next)[])
+    {
+      next[cat] =
+	  addAsChild (next[cat], parentId, picked)
+	  .sort ((a: Tag, b: Tag) => a.name < b.name ? -1 : 1)
+    }
+
+  return next
+}
+
+
 const isDescendant = (
     root:     Tag,
     targetId: number,
@@ -150,10 +204,10 @@ const insertRootAt = (
 }
 
 
-const DropSlot = ({ cat, index }: { cat: Category, index: number }) => {
+const DropSlot = ({ cat }: { cat: Category }) => {
   const { setNodeRef, isOver: over } = useDroppable ({
-      id:   `slot:${ cat }:${ index }`,
-      data: { kind: 'slot', cat, index } })
+      id:   `slot:${ cat }`,
+      data: { kind: 'slot', cat } })
 
   return (
       <li ref={setNodeRef} className="h-1">
@@ -162,12 +216,6 @@ const DropSlot = ({ cat, index }: { cat: Category, index: number }) => {
 }
 
 
-const removeFromRoot = (
-    roots:    Tag[],
-    childId:  number,
-): Tag[] => roots.filter (t => t.id !== childId)
-
-
 type Props = { post: Post | null }
 
 
@@ -189,15 +237,36 @@ export default (({ post }: Props) => {
 
     const overKind = e.over?.data.current?.kind
 
-    if (!(childId) || !(overKind))
+    if (childId == null || !(overKind))
       return
 
     switch (overKind)
     {
     case 'tag':
       const parentId: number | undefined = e.over?.data.current?.tagId
-      if (!(parentId) || childId === parentId)
+      if (parentId == null || childId === parentId)
+	return
+
+      try
+      {
+	if (fromParentId != null)
+	  {
+	    await axios.delete (
+		`${ API_BASE_URL }/tags/${ fromParentId }/children/${ childId }`,
+		{ headers: { 'X-Transfer-Code': localStorage.getItem ('user_code') ?? '' } })
+	  }
+
+	await axios.post (
+	    `${ API_BASE_URL }/tags/${ parentId }/children/${ childId }`,
+	    { },
+	    { headers: { 'X-Transfer-Code': localStorage.getItem ('user_code') ?? '' } })
+      }
+      catch
+      {
+	toast ({ description: '管理者権限が必要です．' })
+
 	return
+      }
 
       setTags (prev => {
 	const child = findTag (prev, childId)
@@ -205,38 +274,33 @@ export default (({ post }: Props) => {
 	if (!(child) || !(parent) || isDescendant (child, parentId))
 	  return prev
 
-	const cat = child.category
-	const next: TagByCategory = { ...prev }
-
-	if (fromParentId)
-	  next[cat] = detachEdge (next[cat], fromParentId, childId)
-	else
-	  next[cat] = removeFromRoot (next[cat], childId)
-
-	next[cat] = addAsChild (next[cat], parentId, child)
+	toast ({ description: `《${ child.name }》を《${ parent.name }》の子タグに設定しました．` })
 
-	return next
+	return attachChildOptimistic (prev, parentId, childId)
       })
 
-      if (fromParentId)
-	{
-	  await axios.delete (
-	      `${ API_BASE_URL }/tags/${ fromParentId }/children/${ childId }`,
-	      { headers: { 'X-Transfer-Code': localStorage.getItem ('user_code') ?? '' } })
-	}
-
-      await axios.post (
-	  `${ API_BASE_URL }/tags/${ parentId }/children/${ childId }`,
-	  { },
-	  { headers: { 'X-Transfer-Code': localStorage.getItem ('user_code') ?? '' } })
-
       break
 
     case 'slot':
       const cat: Category | undefined = e.over?.data.current?.cat
-      const index: number | undefined = e.over?.data.current?.index
-      if (!(cat) || index == null)
+
+      if (fromParentId == null
+	  || !(cat)
+	  || cat !== findTag (tags, childId)?.category)
+	return
+
+      try
+      {
+	await axios.delete (
+	    `${ API_BASE_URL }/tags/${ fromParentId }/children/${ childId }`,
+	    { headers: { 'X-Transfer-Code': localStorage.getItem ('user_code') ?? '' } })
+      }
+      catch
+      {
+	toast ({ description: '管理者権限が必要です．' })
+
 	return
+      }
 
       setTags (prev => {
 	const child = findTag (prev, childId)
@@ -245,18 +309,16 @@ export default (({ post }: Props) => {
 
 	const next: TagByCategory = { ...prev }
 
-	if (fromParentId)
+	if (fromParentId != null)
 	  next[cat] = detachEdge (next[cat], fromParentId, childId) as any
 
-	next[cat] = insertRootAt (next[cat], index, child)
+	next[cat] = insertRootAt (next[cat], 0, child)
+
+	next[cat].sort ((a: Tag, b: Tag) => a.name < b.name ? -1 : 1)
 
 	return next
       })
 
-      await axios.delete (
-	  `${ API_BASE_URL }/tags/${ fromParentId }/children/${ childId }`,
-	  { headers: { 'X-Transfer-Code': localStorage.getItem ('user_code') ?? '' } })
-
       break
     }
   }
@@ -322,7 +384,7 @@ export default (({ post }: Props) => {
 			  <>
 			    {renderTagTree (tag, 0, `cat-${ cat }`, suppressClickRef, undefined)}
 			  </>))}
-		      <DropSlot cat={cat} index={0}/>
+		      <DropSlot cat={cat}/>
 		    </AnimatePresence>
 		  </motion.ul>
 		</motion.div>))}