this problem is fixed around 3hours ago.
App Store Sandbox - Resolved Issue
https://developer.apple.com/system-status/
yeonjeong
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https://buy.itunes.apple.com/verifyReceipt is okay.
https://sandbox.itunes.apple.com/verifyReceipt is returned 21005 from 3rd of December.
I checked by Jenkins every 10 minutes.
I confirmed status code returned 21006 around 9 o'clock JST on today.
This problem was solved. :)