"There is no public key available ..." when apt-get update

I forgot since when, my debian outputs the message like below when executing apt-get update.

W: There is no public key available for the following key IDs:
9D6D8F6BC857C906
W: There is no public key available for the following key IDs:
7638D0442B90D010

This can be fixed by following procedures.

1. apt-get install debian-keyring debian-archive-keyring
2. apt-key update