PURPOSE: To ensure the safety of eyes and keep the distance measuring precision by providing a control means for controlling a timing signal output means in such a manner that the shorter the arithmetically calculated distance, the smaller the frequency of a lighting timing signal.

CONSTITUTION: The pulse beam from a pulse generator 2 is incident directly to a light receiving element 7, and also to a light receiving element 8 through a light receiving convex lens 6 by being reflected by an obstruction through a light transmitting convex lens 5. The delay time of transmitting timing is corrected by use of the element 7. A signal START is outputted from a threshold detector 11 by the light incident to the element 7, and a FF 13 is set. The pulse number during the set of the FF 13 is counted by a counter 14, and the distance to the obstruction is arithmetically calculated. When no pulse beam is incident to the element 8, and a signal STOP is not outputted from a threshold detector 12 within a fixed time, the reciprocating time is not measured, and the absence of the obstruction within a prescribed distance range is judged, and the FF 13 is reset.