Alien spaceships have never entered Japanese airspace, the country's defence minister has told parliamentarians, after being questioned about a possible aerial invasion by little green men.

Gen Nakatani said the nation's war planes can be scrambled whenever there is a report of an unidentified flying object (UFO) but, so far, they had not encountered visitors from space.

"When the Air Self Defence 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," he said.

"They sometimes find birds or flying objects other than aircraft but I don't know a case of finding an unidentified flying object believed to have come over from anywhere other than Earth," he said.

Nakatani, a sober politician with a reputation for thorough understanding of his brief, was responding to a question from flamboyant wrestler-turned-lawmaker Antonio Inoki.

At a meeting Wednesday of the budget committee in Japan's upper house, Inoki had asked if aircraft were dispatched to meet extra-terrestrial visitors and "whether studies (into them) are going on".

Inoki, known for his outsized chin and trademark red scarf, said he did not know whether or not aliens existed, but he had once seen a mysterious flying object rocket into the air on the horizon and disappear.

Wednesday's exchange was not the first official Japanese pronouncement on spacemen and their aircraft.

A 2007 statement 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 at a cabinet meeting at the time, prompted a surprise rebuttal from the top government spokesman.

Nobutaka Machimura, chief cabinet secretary at the time, said: "Personally, I absolutely believe they exist."

Then-defence minister Shigeru Ishiba also said that in his personal opinion there were "no grounds" to deny that there are UFOs controlled by alien life-forms.