I want to know how to check the payment due date (transfer date) online

You can check the payment due date (transfer date) on My au (WEB).

Please see below for more detailed steps.

1
Access
My au (WEB)
on your smartphone or PC.

Login with au ID is required
2

Enter your 4-digit PIN code and tap on "Next" [次へ].
3

The "Charges Breakdown" [料金内訳] page will be displayed.

The payment due date will be displayed directly below the total amount in the section written "Billed for ○○○○ Year ○ Month" [○○○○年○月ご請求].

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