diff --git a/vba_code_kukan_detail_master.txt b/vba_code_kukan_detail_master.txt
index e5301f5..f0034f1 100644
--- a/vba_code_kukan_detail_master.txt
+++ b/vba_code_kukan_detail_master.txt
@@ -40,7 +40,16 @@ Sub FillFromKukanMaster(ByVal ws As Worksheet, ByVal rowNum As Long, Optional By
     On Error Resume Next
     Set wsKukan = ThisWorkbook.Worksheets("区間メンテナンス")
     On Error GoTo 0
-    If wsKukan Is Nothing Then Exit Sub
+    
+    If wsKukan Is Nothing Then
+        Dim wsList As String
+        Dim s As Worksheet
+        For Each s In ThisWorkbook.Worksheets
+            wsList = wsList & s.Name & vbCrLf
+        Next
+        MsgBox "Worksheet '区間メンテナンス' not found." & vbCrLf & vbCrLf & "Available sheets:" & vbCrLf & wsList, vbExclamation
+        Exit Sub
+    End If
     code = Trim(ws.Cells(rowNum, 3).Value)
     If code = "" Then Exit Sub
     lastRow = wsKukan.Cells(wsKukan.Rows.Count, 3).End(xlUp).Row