TOKYO — The police have arrested a man who said he flew a small drone carrying trace amounts of radioactive material onto the roof of the prime minister’s office in Tokyo to protest Japan’s nuclear policy, Japanese news reports said early Saturday.

The unidentified man, who was described only as being in his 40s, turned himself in to the police on Friday in Fukui Prefecture, a rural area on the Sea of Japan that has many nuclear plants. The police had offered no additional details, saying only that they were trying to verify the man’s story, according to the reports.

The arrest, first reported by NHK, Japan’s public broadcaster, indicates that the police have decided to take his confession seriously. He was arrested on charges of obstructing operations at the prime minister’s office.

The discovery of the drone on the roof of Prime Minister Shinzo Abe’s office on Wednesday had baffled officials, especially after the small, propeller-driven craft was found to be carrying a bottle with a liquid containing radioactive cesium. Some authorities feared that the drone was part of a failed act of terrorism aimed at Mr. Abe, who was not in Tokyo when the craft was found.