PURPOSE: To decrease rotation fluctuations of rotary shaft, miniaturize the equipment, and reduce the weight and the number of parts used, by providing a photoelectric speed detector on a belt-wound pulley.

CONSTITUTION: The rotation of a motor 7 is transmitted to a shaft 1 by a belt 10 through a belt-wound pulley 2. This belt-wound pulley 2 is provided with a speed detection hole 3, and a light emitting diode 4 and a photo detector 5 are disposed at the position penetrating through this hole 3. Since the light of the light emitting diode 4 is transmitted and shielded due to the rotation of the pulley 2, the rotating speed may be detected at a high precision by comparing the electric signal from the photo detector 5 with the reference pulse, and, accordingly, the speed of the rotary shaft 1 may be also controlled.