A drone about 50 cm wide equipped with what appeared to be a small camera, a smoke flare and emblazoned with a radiation symbol was found on the roof of Prime Minister Shinzo Abe's Tokyo office on Wednesday morning, officials and media reports said.
Investigators said they also detected trace amounts of radiation from the device.
The drone was discovered at 10:27 a.m. by a worker at the office in the Nagatacho district of Tokyo's Chiyoda Ward, Abe's office said in a statement.
Officials immediately reported the discovery to police, who launched an investigation into the incident, the statement said without elaborating further.
Aerial footage taken by broadcaster NHK on Wednesday showed what appeared to be the drone sitting on top of the five-story building, with investigators milling around the device.
The drone appeared to be equipped with a camera, smoke pot and plastic bottle, NHK reported.
Abe is currently in Indonesia to attend commemorations marking the 60th anniversary of the Asian-African Conference.
The discovery has highlighted an apparent lack of regulations covering drone flights, though various models of the product are now widely available in shops in Japan.
According to the transport ministry, the aviation law requires a drone operator to inform the aviation authority if they plan a flight at an altitude of 250 meters or higher.
However, flights at other altitudes are not covered by the law, except for areas over and near an airport and within the flight routes of other aircraft, ministry officials said.
In January, a quadcopter drone was found inside the White House compound in the United States, making headlines around the world.
Information from Kyodo added
