The dismembered body of what appeared to be a middle-aged or elderly woman's was found Saturday near the Pacific coast in Chiba Prefecture.
A man fishing on a river in the city of Oamishirasato called police in the morning after discovering the decapitated body, police said, adding that the fisherman used his net to pull it out of the water.
The naked torso is that of a person who likely died within the past month, investigators said, adding that the head, arms and feet had been severed after the victim died.
Investigators plan to conduct an autopsy to find out the cause of death and identify the body.
The body was found about 10 km east of Honno Station.