Unable to pay for App Store purchases in Japan with foreign credit card?

A good suggestion, but Apple will often require more than just account balance for making purchases. I see many, many posts about this and presume it would also be Apple Japan's policy too.

Apple's official support articles present multiple statements* about situations requiring the presence of a payment method at times, even for free items. You do not always have to have one, but if you are encountering one of those situations then you have no option but to enter a valid payment method if you wish to continue downloading. The payment method may not be charged if you also have account balance or the item is free, but the payment method has to be present. This next article lists valid payment methods for different countries but note that in this situation Apple Account balance alone apparently may not meet the requirement and you need to use another payment method until you find one that satisfies Apple's requirement (often a credit or debit card): Payment methods that you can use with your Apple ID - Apple Support .

If you have already entered a payment method, use the “Add a payment method” link above to check it. If a message appears in red there is a problem with it. If the information is still correct and current, contact your financial institution. Also check this resource --> If your payment method is declined in the App Store or iTunes Store - Apple Support

* Ref.:

Add a payment method - Apple Support

"To make purchases from the App Store and use subscriptions like iCloud+ and Apple Music, you need at least one payment method on file."

If you can’t download or update apps - Apple Support

- "You might need to have a payment method on file, even if the app that you want to download is free."

Remove a payment method from your Apple ID - Apple Support

- If you have any active subscriptions, you must have at least one payment method on file other than Apple ID credit.

- You might not be able to remove a payment method if you have a balance due.

Sign in with your Apple ID - Apple Support

- "When you sign in to the App Store and other media services for the first time, you might need to add a credit card or other payment information."

How apps, content, and subscriptions from Apple are billed - Apple Support

- You can’t charge gifts to your Apple ID balance. If you send a gift or gift card, your primary payment method is charged.

- Some subscriptions might not be charged to your Apple ID balance.

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

"Some purchases might require that you have a credit card on file, even if you use your Apple Account balance to make the purchase."

You should be able to load it regardless of card status. You may, however, find Apple still asks you for a card for certain purchases. I myself have never been asked for one. IT may depend upon what you are trying to get.

If you do buy something, Apple will try to charge your account balance first in most cases. if there's not enough money, or it's something Apple won't let you charge to account balance, then the card comes into play.

From this support article --> How apps, content, and subscriptions from Apple are billed - Apple Support

"Which payment method is charged?

When you buy items or pay for subscriptions, Apple automatically attempts to charge your payment methods in this order:

1. Apple attempts to use your Apple ID balance to cover the full amount.

2. If you don't have Apple ID balance, you don't have enough to cover the full amount, or an item cannot be paid for with Apple ID balance, Apple attempts to charge your primary payment method (typically a credit or debit card, but varies according to country). The primary payment method is at the top of the list in Settings on your iPhone. You can control which payment method is at the top of the list. [See: Add a payment method - Apple Support ]

3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple may try again." See also sections B and H of Legal - Apple Media Services - Apple

Some purchases aren’t charged to your Apple Account balance

● You can't use your Apple Account balance to send gifts or gift cards.

● For some recurring charges, like AppleCare+, you can't start your subscription with Apple Account balance. But as long as you keep another valid payment method on file and your Apple Account balance would cover an entire charge, your recurring charge will pull from your Apple Account balance (see the article for more information).

● Some subscriptions might not be charged to your Apple Account balance.

● If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are available only to you.