A small drone was found on the roof of the Japanese prime minister's office on Wednesday, apparently having been flown there, local media said.
Staff at the official residence -- known as "the Kantei" -- discovered the 50-centimetre craft on top of the five-storey structure in central Tokyo around mid-morning, major broadcasters reported.
Aerial footage showed dozens of police officers swarming over the roof and helipad of the modern, glass-facade building, with the drone covered by a blue tarpaulin.
Police have not found any explosives on the four-propeller drone, but it did have a camera, a plastic bottle and what appeared to be a flare, NHK said.
Tokyo Metropolitan Police declined to comment on the matter.
Prime Minister Shinzo Abe, who is in Indonesia, works at the building during the day, and commutes from his own private home roughly 15 minutes away.
The use of drones is becoming more common in Japan, particularly for aerial surveying, photography and video shoots. There are currently no legal restrictions on the use of drones.
In January, a hobby drone crashed into the White House gardens, sparking a Secret Service investigation.
The operator was an intelligence agency employee who lost control of his friend's device.
US prosecutors said they would not pursue criminal charges, while the Federal Aviation Administration has begun a review of the mishap for possible administrative action.