To reliably and accurately detect a rotational frequency without effect of mounting accuracy and vibration, by detecting a rotational frequency by a pulley connected through a motor for driving and a belt for power transmission and an optical sensor set on both sides of the pulley.

A set of light emitting element 33a and a light receiving element 33b facing each other set on both sides of a pulley 14 at equal distances are equipped on the tip end of an arm 32 fixed on the outer wall of a clutch 11. For the directivity of light from the light emitting element 33a and the light reception of the light receiving element 33b, wide ones are taken. Six holes 14c are bored on a same circle on the pulley 14 to cross the light axis between the light emitting element 33a and the light receiving element 33b. The more the number of holes, the higher the detecting performance. Since the detecting set judges only whether it received or not, if only the opening area of a hole 14c is sufficiently secured, the detection of a rotational frequency with high reliability can be achieved.