PURPOSE: To raise an S/N ratio and to decide exactly whether a vehicle exists or not, by using a laser diode of a pulse driving type as an illuminating light source, and using a photodetector such as a photodiode, etc., as a detector.

CONSTITUTION: A laser diode 5 is driven by a driver 4 in accordance with turn- on and turn-off of a pulse train 3 from a pulse generator 2. The laser diode 5 oscillates in the time of turn-on of the pulse train 3, and its output light is irradiated toward a reflecting body 16 on a road as transmitting light 7 through a transmitting lens 6. Light reflected from the road surface including the reflecting body 16, out of the transmitting light 7 is received by a photodiode 10 through a receiving lens 9. In accordance with whether a vehicle 1 exists or not on the reflecting body 16, receiving light 8 which is subjected to strength and weakness variations of intensity of light is photoelectrically converted by the photodiode 10, is inputted as a receiving signal to a gate 12, and the gate 12 outputs a detecting signal 13. A deciding device 14 decides whether the vehicle 1 exists or not in accordance with amplitude of the detecting signal 13 is large or small.