To improve the comfortability in riding by discriminating sudden acceleration upon startup before connection of a centrifugal clutch and limiting the engine output power. A supervision starting condition discrimination section discriminates whether or not an engine satisfies a supervision starting condition for starting discrimination of satisfaction of a starting control condition. The supervision starting condition is discriminated based on a vehicle speed, a throttle opening and an engine speed. Particularly, a condition at least of the engine speed from among the parameters such as the vehicle speed is set to a value lower than a speed at which a centrifugal clutch is connected. If the supervision starting condition discrimination section discriminates that the condition is satisfied, then a startup control condition discrimination section discriminates whether or not the vehicle is in a sudden acceleration state based on whether or not the throttle opening has a magnitude (for example, 50%) corresponding to a sudden opening of the throttle. If sudden opening of the throttle is detected, then an ignition retard section issues an instruction to an ignition timing control section to retard the ignition timing in order to limit the output power.