To improve reliability of object monitoring performance of a reflecting-type photo-sensor by generating a signal for indicating safety only when original reflecting light is received in the reflecting-type photo-sensor.

A projector 11 and a light receiver 12 are provided on one side across a monitoring region A, a reflecting plate 13 is arranged on the other end, and the reflecting plate 13 is rotated by a motor 14 at the constant speed. A light beam L from the projector 11 is reflected by the reflecting plate 13 and periodically enters the light receiver 12 in response to the rotational period of the reflecting plate 13. A judging signal for indicating high-level safety is generated through an amplifier 15, an envelope detector 16 and a rectifying circuit 17 only when the light receiving output of the light receiver 12 is the A.C. output having the frequency corresponding to the rotational period of the reflecting plate 13.