Excel VBA 半角カタカナだけを全角カタカナに変換

'**
'* 半角カナを全角カナに変換する関数
'* 引数１：txt {String型} 変換したい文字列を指定
'* 戻り値：{String型} 半角カタカナを全角カタカナに直した文字列
'**
Function Conv_HalfKana_To_FullKana(ByVal txt As String) As String
 
    Dim i As Long
    For i = 1 To Len(txt)
        Dim char As String: char = Mid(txt, i, 1)
        Dim charCode As Long: charCode = AscW(char)
        
        Dim tmpTxt As String
        Dim result As String
        If charCode >= &HFF61 And charCode <= &HFF9F Then
            tmpTxt = tmpTxt & char
        Else
            If tmpTxt <> "" Then
                result = result & StrConv(tmpTxt, vbWide)
                tmpTxt = ""
            End If
            result = result & char
        End If
    Next i
 
    If tmpTxt <> "" Then result = result & StrConv(tmpTxt, vbWide)
    
    Conv_HalfKana_To_FullKana = result
    
End Function