Resolve the BADSIG problem when updating apt list

I recently encounter an update problem during updating sources list:

W: GPG error: SOURCE Release: The following signatures were invalid: BADSIG KEY SENDER

I have no idea how this happened, but have fixed this. First I tried this:

# apt-get clean
# cd /var/lib/apt
# mv lists lists.old
# mkdir -p lists/partial
# apt-get clean
# apt-get update

Well, but no use. So I tried another way:

# apt-get update -o Acquire::http::No-Cache=True

Then it works.