Excelブックのオープン

Public Function OpenBook(filename As String)
    Dim buf As String
    Dim book As Workbook
    Dim openedBook As Workbook

    buf = Dir(filename)
    If buf = "" Then
        Set OpenBook = Nothing
        Exit Function
    End If

    For Each book In Workbooks
        If book.name = buf Then
            Set OpenBook = book
            Exit Function
        End If
    Next book

    Application.ScreenUpdating = False
    Set OpenBook = Workbooks.Open(filename)
    Application.ScreenUpdating = True

End Function