To appropriately prohibit the stop of an engine in a hybrid vehicle system when the vehicle which defines a stop condition and a start condition of an engine based on the power requirement of the vehicle to be used for boat launch.

A controller 60 of the hybrid vehicle system 10, when using the vehicle for boat launch, determines engine stop/start conditions based on the power requirement of the vehicle, determines whether the vehicle is used for boat launch for carrying a boat in a shore and move it on water, determines whether a shift position is in a moving back position, and determines whether the current position of the vehicle is in the shore and the application of the engine stop condition is prohibited based on these determination results.