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 delay time setting component sets a delay time according to the acceleration data. A motor controller supplies the motor 13 with a magnitude of electricity in accordance with the acceleration operation amount after the delay time has elapsed from the moment when an accelerator grip was moved from an idle position.