Clutch operation signal issued in operational association with depression of a clutch pedal are taken as answer-back signals into an electronic control unit (ECU), a starter start command signal is issued in response to one of these answer-back signals. Additionally, a clutch upper switch and a clutch lower switch, which constitute a clutch sensor and are connected in series to each other, are inserted into a starter relay driving system, so that start due to mistaken operation and the like of ECU can be avoided.