A vehicle driven by an internal combustion engine incorporating a power supply unit and including a power transmission on/off determination device that gradually opens a throttle valve of the engine to gradually increase a rotational speed of the engine, detects, as a determination value that reflects a load applied on the internal combustion engine when the speed of the engine increases to a power transmission start rotational speed, and then the throttle valve is further opened by a set angle, determines that the power transmission device is in a state of cutting the power transmission when it is determined from the determination value that the engine load is less than a preset magnitude, and determines that the power transmission device is in a state of performing the power transmission when it is determined from the determination value that the engine load is equal to or more than the preset magnitude.