Four others were hurt in the blaze on Wednesday, with terrified residents forced to jumped out of upper-floor windows of the five-storey block in the 18th district of the French capital as flames engulfed the building.
Judicial sources told AFP the suspect -- who was arrested near the scene -- had a history of mental illness and was admitted to a psychiatric hospital for a time after police began questioning him.
Firemen were initially called to deal with a small fire at the building in the Goutte d'Or area of the city, which has a high emigrant population, before the fatal blaze broke out two hours later.
Six of the victims died of smoke inhalation, including two children aged eight and 14, a 44-year-old woman and a man of 63, all apparently from the same family, officials said.