Police say the other 38 passengers and the driver were injured in Sunday morning's accident on a highway in Gunma, north of Tokyo.
The bus crashed into a roadside wall. Steel wall panels sliced the vehicle in half.
Police are investigating the cause. They say all the passengers are believed to be Japanese nationals, mostly young adults.
Police were having difficulty identifying the bodies. Six of those killed were women.
The crash happened as Japan head into the "Golden Week" holidays this week.
