PURPOSE: To enable to miniaturize a rotation speed sensor and perform a high-reliable operation through transmission of a signal by use of an optical fiber, by a method wherein a shaft for switching an optical path is intermitently driven through a permanent magnet which monolithically turns togeth-with a rotary object.

CONSTITUTION: Permanent magnets 16, being different in polarity and located facing and opposite to each other in a position being 180° different from each other, are mounted to a disc 17 secured to a rotary shaft 17. An intermittent drive of a rotary disc 22 in an enclosure receptacle 18 is regulated through the magnets 16, and a recess 28, through which a light flux, mounted to a shaft 21 monolithically united with the rotary disc 22, is caused to pass, and a reflecting surface, reflecting the light flux, are positioned facing and opposite to optical fibers 23 and 24 connected to a light source 25 and a light signal processor 27, respectively, corresponding to a rotation speed. Through switching of an optical path corresponding to the speed, a photo pulse through the fiber 23 is emitted to the fiber 24, and the rotation speed is detected by means of a signal transmitted to an optical fiber which eliminates the need for a shield against an electromagnetic noise, and a rotation sensor is miniaturized and operates with a high precision.