PURPOSE: To detect only bodies within a prescribed detection angle range and to detect none of such short animals as a dog by providing sensors which detect a far infrared ray emitted by a human body at prescribed height.

CONSTITUTION: The far infrared ray sensors 2 and 3 are fixed to fixing angle steel pieces nearby the upper end of a housing 1. The sensor 2 is fitted with a spherical mirror 2a as part of a spherical surface with a radius R, a cylinder 2b which has the same diameter as that of the spherical mirror 2a and length R/2, and a pyroelectric infrared ray detecting element 2c whose detection surface faces the spherical surface at a position where the far infrared ray is condensed through the spherical mirror 2a fitted to the top of the cylinder 2b. Which of the sensors 2 and 3 generates a signal first depends upon the moving direction of the human body. Consequently, slanting far infrared rays are intercepted by the spherical mirror 2a and cylinder 2b to detect only bodies within the front narrow detection angle range, and none of such short animals as a dog and a cat is detected.