PURPOSE: To improve the durability of a starter motor and reduce the power consumption of a battery by additionally attaching a ratchet mechanism to one of gears from the starter motor to a crankshaft to regulate the reversal of the gear in the stoppage of an engine.

CONSTITUTION: When an engine is stopped, a piston connected to a crankshaft 24 cannot pass through the compression stroke and tends to rotate an idle gear 16 in the opposite direction to the arrow C to reverse the crankshaft 24, while a claw 34 engages the idle gear 16 to block the reversal of the crankshaft 24. Thus, the movement of the piston is stopped on the way of the compression stroke. And when a starter motor 12 is driven again to start the engine, the piston ascends on the way of the compression stroke to move to the upper dead point of compression. In this case, however, a driving force needed to rotate the crankshaft 24 is small. Thus, the durability of the starter motor is improved and the power consumption of a battery is reduced.