"I know that this will make many people worry, but we must take note of the fact that his stay was only one month," Tetsuyuki Kitamoto, a Tohoku University professor and head of the ministry panel on the disease, told a news conference.

Kitamoto said he could not rule out the possibility that the man had contracted the disease in Japan because, on a medical basis, nothing could be entirely ruled out.

More than 160 people, most of them in Britain, have died worldwide from definitive or probable vCJD after eating meat contaminated with mad cow disease, formally known as bovine spongiform encephalopathy (BSE).

Britain has been the worst hit by BSE, which is thought to be transmitted among animals via feed containing bovine brains or spinal cord.

About 7 million animals had been slaughtered in Britain by the end of June 2004 under a plan to stop the spread of the infection.

Japan has reported 14 cases of BSE and began testing all its cattle for the disease after the first case in September 2001.

It banned imports of Canadian beef in May 2003 and of US beef in December 2003 after cases of mad cow disease were found in those countries. It is in drawn-out talks on when to lift the ban.

Cases of vCJD have also been reported in France, Canada, Ireland, Italy, the United States and China, Health Ministry officials said.

In all cases outside Europe, victims are believed to have contracted the disease during stays in Britain, but a one-month period would be the shortest stay reported so far, the experts on the health ministry panel said.

The Japanese man, who was in his 40s when he first showed symptoms of the disease in December 2001, had no record of blood transfusions or brain surgery -- other ways in which the disease could be transmitted.

The Health Ministry sought to calm fears among the Japanese public, issuing a statement saying the disease is not transmitted among humans under regular living conditions.

Doctors on the panel said people could consult physicians, but added that, at present, there was no way to determine whether a person would show symptoms, or to stop the progress of the disease.

Scientists estimate the incubation period for vCJD is 10 to 20 years.