root: # apt-get update

Get:1 http://security.debian.org sarge/updates Release.gpg [197B]
Get:2 http://security.debian.org testing/updates Release.gpg [197B]
Ign http://www.calno.com unstable Release.gpg
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Hit http://non-us.debian.org sarge/non-US/contrib Packages
Hit http://non-us.debian.org sarge/non-US/non-free Packages
Fetched 194B in 14s (14B/s)
Reading package lists... Done
W: GPG error: http://www.os-works.com testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY CF455A0A8AC2C0A6
W: You may want to run apt-get update to correct these problems





WELL ~~ LET'S GOOGLE

THE ANSWER AS FOLLOWING:

root: # gpg --keyserver wwwkeys.eu.pgp.net --recv-keys CF455A0A8AC2C0A6
root: # gpg --armor --export CF455A0A8AC2C0A6 | apt-key add -
root: # apt-get update

It's done!! ^^