To surely detect the existence of vehicle in the distance without hurting men abiding in the neighborhood by measuring the distance by scanning in which light emission in the direction where a target is detected within a short distance, is terminated, and light emission in the direction no target is detected in a short distance, is increased in power.

A light beam radiated from a light emission part 1 is radiated via a rotational mirror 5, reflected by a vehicle and the like in the front, received with a light reception part 6 and distance value is calculated from the light reception signal with a distance operator part 7. In preliminary scanning distance measurement with a small light emission power, the light emission direction and the measured distance value in that direction are stored in a preserving memory for the emission angle and the measured distance value. Then, in this scanning distance measurement, memory contents are referred to and light emission is controlled to terminated in the direction a target is detected within a short distance and to increase the emission power in the direction where no target is detected in a short distance.