PURPOSE: To obtain a compact and useful sensor, which projects light toward a very close point and a distant point with high illuminace and low illuminace in an uneven made, by readily assembling and disassembling housings, which surround the sensor parts, and providing ventilating openings at rational positions, which do not interfere with the functions of the sensor.

CONSTITUTION: A sensor comprises a light receiving and detecting part 1 and a light projecting part 2. The respective parts are surrounded by housings 3 and 4. Both housings are stacked and coupled so that they are readily disassebled. The light projecting part 2 is provided with an iodine lamp so that a visual light beam is projected toward a very close point at high illuminance. The surface of a parabolic mirror is made rough surface so that illuminance unevenness on the side of a body, which passes the very close point, is in the minimum allowable limit. Thus the reflected light other than parallel light beams is applied. The housing 3 and a frame 26 are freely communicated to free air at the bottom surface. The housing 4 at the upper part can be communicated to free air freely through many ventilating holes 49 in an intermediate frame 47 at the lower part.