|The bus strayed onto the wrong side of the road and smashed through a guardrail|
The bus, with 41 people on board, was travelling overnight from Tokyo to a resort in Nagano prefecture.
The accident happened near the town of Karuizawa early on Friday.
It is not immediately clear what caused the crash as there was no snow or ice on the road. Japan's transport ministry has launched an investigation.
Twenty-seven people were injured when the bus strayed onto the wrong side of the road and smashed through a guardrail, sliding several metres down the mountainside.
There were two bus drivers on board, taking turns to drive, according to AP news agency.
Skiing and other winter sports are popular in Japan, which has extensive mountain ranges accessible from many of the country's biggest cities. Many people heading to the mountains use a bus for at least part of their journey.