The bus was carrying 41 people to popular ski resort Kita Shiga in Yamanouchi Town when it crashed near the town of Karuizawa, Nagano Prefecture, according to local police. Twenty-seven people were injured.
It's not known what caused the accident; images from the scene showed no snow on the road and conditions were dry at the time of the crash, police said.
The bus operating company has a record of drivers' safety violations and is now under investigation, said ministry officials. It had received a penalty and vehicle suspension only two days previously.