A letter containing two bullets and addressed to Social Democratic Party leader Mizuho Fukushima and her predecessor, Takako Doi, was discovered at the Tokyo central post office Friday.
Police said they detected the bullets during an X-ray test on the envelope.
The SDP, a minor opposition party, had refused to accept the letter because the sender was not specified on the envelope, postmarked Thursday in Yokohama.
A letter containing bullets was sent to the SDP in November 2002.