diff --git a/backend/app/services/post_import_runner.rb b/backend/app/services/post_import_runner.rb
index 2641b6b..d21c3e5 100644
--- a/backend/app/services/post_import_runner.rb
+++ b/backend/app/services/post_import_runner.rb
@@ -18,6 +18,7 @@ class PostImportRunner
   private
 
   def run_row row
+    validate_row_structure!(row)
     attributes = row.fetch('attributes', { }).to_h.transform_keys { _1.to_s.underscore }
     raise ArgumentError, '取込項目が不正です．' unless (attributes.keys - ATTRIBUTE_KEYS).empty?
     raise ArgumentError, '取込項目が大きすぎます．' if attributes.values.any? { _1.to_s.bytesize > PostImportSourceParser::MAX_CELL_BYTES }
@@ -57,4 +58,16 @@ class PostImportRunner
     raise ArgumentError if sources.values.any? { _1.bytesize > PostImportSourceParser::MAX_CELL_BYTES }
     raise ArgumentError if sources.values.sum(&:bytesize) > PostImportSourceParser::MAX_CELL_BYTES
   end
+
+  def validate_row_structure! row
+    provenance = row['provenance']
+    raise ArgumentError unless provenance.is_a?(Hash)
+    allowed = ATTRIBUTE_KEYS + ['url']
+    raise ArgumentError unless (provenance.keys - allowed).empty?
+    raise ArgumentError unless provenance.values.all? { _1.in?(['automatic', 'mapped', 'fixed', 'manual']) }
+    metadata_url = row['metadata_url']
+    raise ArgumentError unless metadata_url.nil? || metadata_url.is_a?(String)
+    raise ArgumentError if metadata_url.to_s.bytesize > PostImportSourceParser::MAX_CELL_BYTES
+    raise ArgumentError if row.to_json.bytesize > PostImportSourceParser::MAX_BYTES
+  end
 end
diff --git a/backend/app/services/post_import_source_parser.rb b/backend/app/services/post_import_source_parser.rb
index 999cb1c..a36b8c5 100644
--- a/backend/app/services/post_import_source_parser.rb
+++ b/backend/app/services/post_import_source_parser.rb
@@ -44,7 +44,7 @@ class PostImportSourceParser
     keys = value.grep(Hash).flat_map(&:keys).map(&:to_s).uniq
     { columns: keys.each_with_index.map { |key, index| { key:, label: "#{ ('A'.ord + index).chr }：#{ key }" } },
       rows: value.each_with_index.map { |item, index| { source_row: index + 1,
-                                                         values: keys.map { item.is_a?(Hash) ? item[_1] || item[_1.to_sym] : nil } } } }
+                                                         values: keys.map { json_cell(item.is_a?(Hash) ? item[_1] || item[_1.to_sym] : nil) } } } }
   rescue JSON::ParserError, KeyError, ArgumentError => e
     raise ArgumentError, "JSON の形式が不正です: #{ e.message }"
   end
@@ -53,4 +53,13 @@ class PostImportSourceParser
     raise ArgumentError, "取込件数は #{ MAX_ROWS } 件までです．" if rows.length > MAX_ROWS
     raise ArgumentError, 'セルが大きすぎます．' if rows.flatten.any? { _1.to_s.bytesize > MAX_CELL_BYTES }
   end
+
+  def json_cell value
+    case value
+    when nil then ''
+    when String, Numeric, TrueClass, FalseClass then value.to_s
+    when Array, Hash then JSON.generate(value)
+    else raise ArgumentError, 'JSON のセル値が不正です．'
+    end
+  end
 end
diff --git a/frontend/src/pages/posts/PostImportPage.tsx b/frontend/src/pages/posts/PostImportPage.tsx
index 4b5ad89..084150a 100644
--- a/frontend/src/pages/posts/PostImportPage.tsx
+++ b/frontend/src/pages/posts/PostImportPage.tsx
@@ -32,6 +32,7 @@ type ImportRow = {
     status: 'ready' | 'warning' | 'error'
     existing: boolean
     metadataUrl?: string
+    createdPostId?: number
     importStatus?: 'pending' | 'created' | 'skipped' | 'failed' }
 type ParsedImport = { columns: Column[], rows: { sourceRow: number, values: string[] }[] }
 type Phase = 'source' | 'mapping' | 'preview' | 'result'
@@ -75,6 +76,10 @@ const PostImportPage: FC<Props> = ({ user }) => {
         existing })
       setColumns (data.columns)
       setParsed (data)
+      setMappings ({ })
+      setRows ([])
+      setResults ([])
+      setUrlColumn ('')
       if (data.columns.length === 1)
         setUrlColumn ('0')
       else
@@ -175,6 +180,24 @@ const PostImportPage: FC<Props> = ({ user }) => {
     }
   }
 
+  const updateExisting = async (value: 'skip' | 'error') => {
+    setExisting (value)
+    const generation = ++validationGeneration.current
+    setLoading (true)
+    try
+    {
+      const validated = await apiPost<{ rows: ImportRow[] }> ('/posts/import/validate',
+                                                                { rows, existing: value, changed_row: -1 })
+      if (generation === validationGeneration.current)
+        setRows (validated.rows)
+    }
+    finally
+    {
+      if (generation === validationGeneration.current)
+        setLoading (false)
+    }
+  }
+
   const submit = async () => {
     const targets = rows.filter (row => (row as ImportRow & { importStatus?: string }).importStatus !== 'created')
     if (targets.length === 0)
@@ -192,7 +215,9 @@ const PostImportPage: FC<Props> = ({ user }) => {
       setResults (result.rows)
       setRows (current => current.map (row => {
         const resultRow = result.rows.find (_1 => _1.sourceRow === row.sourceRow)
-        return resultRow ? { ...row, importStatus: resultRow.status } : row
+        return resultRow ? { ...row, importStatus: resultRow.status,
+                             createdPostId: resultRow.post?.id,
+                             errors: resultRow.errors ?? row.errors } : row
       }))
     }
     catch
@@ -273,7 +298,7 @@ const PostImportPage: FC<Props> = ({ user }) => {
 		<label>URL 列 <select value={urlColumn} onChange={ev => setUrlColumn (ev.target.value)}>
 		  {columns.map ((column, index) => <option key={column.key} value={index}>{column.label}</option>)}
 		</select></label>
-		<label>既存投稿 <select value={existing} onChange={ev => setExisting (ev.target.value as typeof existing)}>
+        <label>既存投稿 <select value={existing} onChange={ev => updateExisting (ev.target.value as typeof existing)}>
 		  <option value="skip">スキップ</option><option value="error">エラー</option>
 		</select></label>
 		{['title', 'thumbnail_base', 'original_created_from', 'original_created_before', 'duration', 'tags', 'parent_post_ids'].map (field =>
@@ -290,6 +315,7 @@ const PostImportPage: FC<Props> = ({ user }) => {
 	    <div className="space-y-3"><p>インポート処理が完了しました。</p>
 	      {results.map (result => <div key={result.sourceRow} className="rounded border p-2 dark:border-neutral-700">行 {result.sourceRow}：{result.status} {result.post && <PrefetchLink to={`/posts/${result.post.id}`}>投稿を開く</PrefetchLink>}<FieldError messages={Object.values (result.errors ?? { }).flat ()}/></div>)}
 	      <Button type="button" onClick={() => setPhase ('preview')}>結果を確認</Button>
+	      <Button type="button" variant="outline" onClick={() => setPhase ('preview')}>失敗行を修正</Button>
 	      <Button type="button" variant="outline" onClick={() => setPhase ('source')}>新しい入力元を選ぶ</Button>
 	    </div>)}
 	</MainArea>)