An intruder threw a 12-year-old girl from the second-floor balcony of her house early Friday in Showa, Saitama Prefecture, police said.
Minami Tsukamoto sustained hip and knee injuries. Police said they are investigating the incident as an attempted murder.
Officers are searching for the assailant, whom the victim described as a short, slim man wearing a cap, green T-shirt and black trousers.
Police said they believe a bamboo screen below the balcony of the town house cushioned her fall.
The girl was sleeping when the man entered at around 4:20 a.m. He grabbed her and threw her off the balcony without uttering a word, according to police.
Nothing was stolen during the bizarre occurrence.
Four people in the girl's family found her after hearing a scream. They had not locked their doors or windows Thursday night.