By Agence France-Presse
The vehicle fell around 1:00 pm (0400 GMT) from the parking garage in Yokosuka city, south of Tokyo, according to media witnesses' accounts.
Television footage showed a metal fence guarding the fifth floor of the garage was cut, apparently falling to the ground with the silver vehicle.
"The noise was like thunder. The fence came down and the vehicle came down," a witness told national broadcaster NHK.
The accident killed a man in his 50s, a woman in her 40s and another woman in her 70s, while a man in his 20s and an 11-year-old boy suffered serious injuries, according to private Nippon Television.
They were passengers of the vehicle, Japanese media said.
Local police and rescue officials could not immediately comment on the case.