PURPOSE: To facilitate the start of an engine by transmitting a rotational driving power of a starter motor together with a driving of a starter motor to a supercharger at the time of the engine start.

CONSTITUTION: When a start switch is closed, a shaft 3 of a starter motor 1 is projected while rotating and a gear 2 fixed on the top end of the shaft 3 is engaged with a gear 6 of a supercharger 4. Then, it is engaged with a ring gear R provided on a periphery of a flywheel F. A rotation of the starter motor 1 is transmitted to one side, the gear 6 and the rotating shaft 5 to drive the supercharger 4. Nearly at the same time, before, or after, it is transmitted to the output shaft of the engine via another ring gear R and the flywheel F.