A fire broke out in a five-story apartment building in Omuta, Fukuoka Prefecture, on Sunday morning.
It burned the rooms in which two families lived and left six people, including children injured, police said.
They said the fire broke out at around 6:15 a.m., probably in the third-floor room of Etsuko Kawamoto, 52, who was taken to hospital. Tomohiro Hashiguchi, 31, who lives above Kawamoto, was also hospitalized along with his wife Mami, 32, and their sons Toma, 11, Akihiro, 7, and Kengo, 2. A 10-year-old son was also at home but managed to escape safely.
Omuta police and fire department officials are investigating the cause of the fire, which was extinguished after about 2½ hours.
The apartment building, which is operated by the city, was built more than 40 years ago and the city was planning to pull it down and rebuild it.
Police officials are also investigating whether the fire alarms installed at kitchens and bedrooms were working properly.