Police quoted the suspect as saying he did it because "I was tired of living," public broadcaster NHK reported.
The blaze broke out at about 3 a.m. (1800 GMT) in the Cats Adult Video Theater, located on the ground floor of a seven-story building in an entertainment district in Japan's second-largest city, fire department official Yuko Kotani said.
Hirami said officials were still trying to determine the identities of the 15 dead. Several of the injured, nine men and one woman, were in a serious condition in hospital.
When the fire started, most of the theater's 32 viewing cubicles - each equipped with a cot, a television and a DVD player - were occupied.
The video center offered a wide selection of DVDs, mostly adult movies, but also cartoons and Hollywood films, and attracted businessmen and others who used the rooms as a cheap alternative to hotels after missing the last train home.
A growing number of young people whose low-paying jobs cannot pay for proper housing live in such all-night lounges watching videos or reading comic books.
The Ministry of Land, Infrastructure, Transport and Tourism instructed fire department officials Wednesday to conduct an emergency nationwide inspection of video shops and Internet cafes, as well as karaoke bars and other entertainment facilities with individual cubicles.
In 2001, a fire in Tokyo's biggest red-light district killed more than 40 people, including bar hostesses and their customers.