Bullets were fired at the home of a secretary of posts minister Taro Aso in Toshima Ward, Tokyo, early Monday, police said.
Investigators said the wife of Ichiro Muramatsu called to report that she had discovered four holes on the front door of her house at around 6:50 a.m.
Two of the bullets apparently passed through a glass part of the door and the others had pierced an aluminum section. Three bullets were found embedded in the wall of a first-floor room. No injuries were reported.
The 48-year-old Muramatsu, who lives in the house with his wife and son, said he heard popping sounds at around 1:45 a.m., Tokyo police said.
Aso, 63, is a senior lawmaker of the Liberal Democratic Party and serves as minister of public management, home affairs, posts and telecommunications.
Muramatsu has been serving as his secretary for 18 years, and became his political affairs secretary when Aso became a Cabinet minister in September.
Aso told reporters that Muramatsu had called him in the morning to tell him of the incident, but that he had no idea why such a thing should happen.
Chief Cabinet Secretary Yasuo Fukuda said at his daily news conference that the government will beef up the police investigation.
"I don't know why such a thing happened, so it's difficult to comment," he said. "But firing a handgun (into a residence) is unforgivable under any circumstances."