diff --git a/vba_code_kukan_detail_master.txt b/vba_code_kukan_detail_master.txt
index b0d2657..a275bbd 100644
--- a/vba_code_kukan_detail_master.txt
+++ b/vba_code_kukan_detail_master.txt
@@ -1,3 +1,6 @@
+' CSV Header Constants
+Const CSV_HEADER As String = "利用区間コード,券種,コード,名称,1箇月運賃/販売額,定期額/券１(額)/利用額,定期支給期間/券１(枚)/特別料金,特別料金/券２(額),券２(枚),端数(額),特別料金"
+
 Private Sub Worksheet_Change(ByVal Target As Range)
     ' === Generate F dropdown based on G column (matching left of colon) ===
     If Target.Column = 7 And Target.Row >= 7 Then
@@ -169,6 +172,16 @@ Sub ImportMasterDetailData()
     
     lines = Split(textContent, vbCrLf)
     
+    ' === Validate CSV header ===
+    If UBound(lines) >= 0 And Trim(lines(0)) <> "" Then
+        Dim csvHeader As String
+        csvHeader = Trim(lines(0))
+        If csvHeader <> CSV_HEADER Then
+            MsgBox "CSV header mismatch." & vbCrLf & "Expected: " & CSV_HEADER & vbCrLf & "Got: " & csvHeader, vbExclamation
+            Exit Sub
+        End If
+    End If
+    
     ' === Clear all data rows before import ===
     lastRow = wsTarget.Cells(wsTarget.Rows.Count, "C").End(xlUp).Row
     If lastRow >= 7 Then
@@ -368,9 +381,9 @@ Sub ExportMasterDetailData()
         savePath = savePath & ".csv"
     End If
     
-    ' Header:,1箇月運賃/販売額,定期額/券１(額)/利用額,定期支給期間/券１(枚)/特別料金,特別料金/券２(額),券２(枚),端数(額),特別料金
+    ' Use constant for header
     Dim headerList As Variant
-    headerList = Array("利用区間コード", "券種", "コード", "名称", "1箇月運賃/販売額", "定期額/券１(額)/利用額", "定期支給期間/券１(枚)/特別料金", "特別料金/券２(額)", "券２(枚)", "端数(額)", "特別料金")
+    headerList = Split(CSV_HEADER, ",")
     
     ' Build CSV
     Dim csvContent As String