Veteran Liberal Democratic Party lawmaker Hiromu Nonaka received a 5.5 cm piece of metal in the mail that might be a bullet at his Tokyo office, police said Thursday.
The sender is believed to be the same group that claimed responsibility for a bomb that was planted at a top Foreign Ministry official's home the previous day, investigators said.
Nonaka, a former LDP secretary general, announced earlier in the week that he will not run for office again after his current term ends.
The sender was identified on the envelope as the ultrarightist group Kenkoku Giyugun (nation-building volunteer corps).
On Wednesday, Kenkoku Giyugun claimed responsibility for planting a bomb at the home of Deputy Foreign Minister Hitoshi Tanaka, a key official in negotiations and policymaking on North Korea.
The white envelope was posted around 2:20 p.m. Thursday to Nonaka's office in a House of Representatives building in Tokyo's Nagata-cho district.
A Tokyo post office employee detected a metal object in the envelope during screening and conveyed this information to Nonaka when the envelope was delivered.
The unopened envelope was handed to police officials, who discovered what looked like a bullet inside. The piece of metal is 4.6 mm in diameter, they said.
Nonaka told reporters that he regretted the "existence of such forces when (the country) is aiming to be a peaceful nation."
He vowed to fight them, saying his "position and conviction seeking peace and friendship in Asia will remain undaunted" by the challenge posed by such elements.
Prime Minister Junichiro Koizumi urged a thorough probe, saying the government needs to seriously deal with threats and incidents that hinder political activities.
Chief Cabinet Secretary Yasuo Fukuda condemned the incident.