The bodies of the 35-year-old woman and three youngsters - a seven-year-old boy, a girl aged six, and a two-year-old boy - were found near the entrance of the 14-storey building in Chiba, east of Tokyo, a police spokesman said.
"They are believed to be a mother and her children," he said. Asked if it appeared to be a forced family suicide, he said: "Police are investigating the case with that possibility under consideration."
Despite a high-quality healthcare system universally available, relatively few people with psychological problems seek treatment in Japan, where mental illness retains its stigma.