Police arrested a 27-year-old man Saturday on suspicion of assaulting his girlfriend's 4-year-old son, resulting in the boy's death.
Ryuji Kawabe beat the boy, Hayato Narita, in the face and abdomen Friday afternoon at his apartment in Tokyo, the police said.
The boy was taken to a hospital in the evening after his 22-year-old mother phoned for an ambulance.
"I beat him because he didn't eat his lunch," Kawabe allegedly told the police. The mother was reportedly at a coin laundry at the time. The police said an autopsy would be performed.