An automatic centrifugal clutch is interposed in a power transmission system between an engine and a driving wheel. A motor, which is capable of generating electricity, is supplied with electricity from a battery to generate auxiliary power. The motor is connected to a crankshaft of the engine. An acceleration data acquisition component acquires the accelerator operation amount and the accelerator operation speed. A rotational speed detection component detects the speed of the crankshaft. A rotational speed estimation component is adapted to estimate the engagement completion rotational speed, which is a rotational speed at which the automatic centrifugal clutch is completely engaged, based upon the acceleration data acquired by the acceleration data acquisition component. A motor controller supplies the motor with a magnitude of electricity in accordance with the acceleration data when the rotational speed detected by the rotational speed detection component has reached the engagement completion rotational speed estimated by the rotational speed estimation component.