The country's defence minister told parliamentarians that Japan's air force had been scrambled over unidentified flying objects several times but none had turned out to be anything other than terrestrial.
General Nakatani made the statement after being questioned about the possibility that beings from outer space had ever tried to invade Japan.
He said: "When the Air Self-Defense Force detects indications of an unidentified flying object that could violate our country's airspace, it scrambles fighter jets if necessary and makes visual observation.
"They sometimes find birds or flying objects other than aircraft but I don't know of a case of finding an unidentified flying object believed to have come from anywhere other than Earth."
The General, a sober politician with a reputation for thoroughly understanding his brief, was responding to a question from wrestler-turned-politician Antonio Inoki.
At a meeting of the budget committee in the Japanese parliament's upper house on Wednesday, Inoki asked whether aircraft had been sent to meet extra-terrestrial visitors and if "studies (into them) are going on".
Mr Inoki said he did not know whether aliens existed or not, but he had once seen a mysterious flying object shoot into the air on the horizon and disappear.
Wednesday's reply was not the first time the Japanese government has made a comment on aliens and the spaceships they travel in.
A statement in 2007 said the government "has not confirmed the existence of unidentified flying objects believed to have come from anywhere other than Earth".
But the statement, formally endorsed by the cabinet at the time, prompted senior officials to make surprising admissions.
Nobutaka Machimura, chief cabinet secretary, said afterwards: "Personally, I absolutely believe they exist."
Then-defence minister Shigeru Ishiba added that in his personal opinion there were "no grounds" to deny that there are UFOs controlled by alien life-forms.