PURPOSE: To enhance the utilization efficiency of light from a light projection means, in an apparatus wherein the light projection means and a light receiving means are provided so as to hold the slit body cooperated with an object to be detected therebetween, by providing a light reflective plate at a part not opposed to the slit of the light projection means.

CONSTITUTION: A slit disc 20 having a slit 22 is connected to an object to be detected by a shaft 21 and a light projection means 30 and a light receiving means 40 are arranged in opposed relation to each other so as to hold the slit disc 20 therebetween. The slit disc 20 is rotated in connection with the rotation of the object to be detected and, when an angle of rotation is detected from the position of light inputted to the light receiving means 40 through a slit 2, the residual part not opposed to the slit 22 of the front surface 30c of the light projection means 30 is covered with a light reflective plate 34. By this method, because the light emitted from the light projection means 30 entirely irradiates the part opposed to the slit 22, efficiency is enhanced and the apparatus can be miniaturized.