The fire broke out on the fourth or fifth floor of an eight-story building in a shopping and entertainment area in the city of Osaka on Friday, NHK said.
A total of 28 people were “severely injured”, it said, with “most of them without vital signs.”
An official at Osaka city’s fire department told Reuters that 27 people were in cardiopulmonary arrest, the term used in Japan before a death is officially confirmed.
TV Asahi said nine people have been confirmed dead, Al Jazeera reported.
The fire was reported shortly after the clinic opened for business at 10am (0100 GMT) and was mostly extinguished within 30 minutes, NHK reported.
Videos broadcast on the network showed smoke pouring out of the building’s fourth floor windows, where a psychiatry clinic is located, and at the roof of the building.
Police sources told Kyodo News they are investigating suspected arson, including reports that a man started a fire in the building.
MA/PR