When a system stop instruction is issued in the middle of the operation of an engine to stop the engine, the engagement of the gears in a gear mechanism is pressed toward one side. After the engine has stopped, a torque command for a motor connected to a drive shaft is gradually reduced. When the torque command decreases below a threshold value, the system is stopped. In this way, it is possible to prevent generation of vibration of the drive shaft when the system is stopped.