To decrease a weakness of light, in a reflective object detecting system comprising a light projector, a reflector reflecting a light projected from the light projector and a light receiver receiving a light reflected from the reflector.

A pulse light is projected from a light projector 1. A reflector 2 varies a reflectivity optionally and reflects the pulse light from the light projector 1 by amplitude modulation. The reflector 2 can constitute a reflective liquid-crystal device having a reflectivity varied by a conversion of an applied voltage. Accordingly, an electric signal is obtained from the light receiver 3 by a photo-electricity conversion, and the presence or absence of an object is detected by detecting a frequency or pulse width of the electric signal.