GPG error : http://archive.ubuntulinux.jp

After add the following to /etc/apt/sources.list

deb http://archive.ubuntulinux.jp/ubuntu-ja hardy/

and run

apt-get update

You will be prompted:

W: GPG error: http://archive.ubuntulinux.jp hardy/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 058A05E90C4ECFEC
W: You may want to run apt-get update to correct these problems

Just ignored it. Run

apt-get install ubuntu-ja-keyring

ignore the warning

WARNING: The following packages cannot be authenticated!
ubuntu-ja-keyring
Install these packages without verification [y/N]? y

just run

apt-get update

again. There will no warning prompted again.