PURPOSE: To form a starter device in a compact size further reduce the number of its required part items, by forming a reduction gear mechanism combined with a one-way clutch comprising a steel ball, conical wheel, fixed plate, carrier having a half moon shaped groove with a one-way slope and a leaf spring.

CONSTITUTION: When a starter device is not operated, a carrier 27 and a pinion gear 26 are pressed to an electric motor 1 by a leaf spring 31, and a steel ball 24 is brought in contact with the deepest part of a half moon shaped groove 27a. Next, if an electric motor 1 is started by closing a starter switch, the pinion gear 26 is engaged with a ring gear in an engine side by moving the steel ball 24 along a space between a sloped face of the half moon shaped groove 27a and a conical wheel 21 so as to move the carrier 27 against the leaf spring 31. Here the electric motor drives an output shaft 25 while a speed of the motor is reduced by a planetary motion of the steel ball 24, by adapting the carrier 27 to a stopper 32. Then the starter device functions as a one-way clutch by moving the steel ball 24 to a side of the deepest part in the half moon shaped groove 27a if a speed in the engine side increases. In this way, the starter device is formed in a compact size while the number of its required part items can be decreased.