PROBLEM TO BE SOLVED: To let an invader hardly visually notice a detector by constituting the detector so that the front face of a regressive reflection mirror formed out of a material transmitting near infrared rays and obstructing transmission of visual rays presents a black color or a dark color similar to it. ;SOLUTION: This detector is composed of a light projecting part 1 having a light projecting element (a) of near infrared rays, a light projecting-receiving device 3 housing a light receiving part 2 having a light receiving element (b)of near infrared rays, and a reflector 4 stuck with a regressive reflection mirror 5 on a fitting base 6. The regressive reflection mirror 5 is formed out of a material favorably transmitting near infrared rays R and obstructing transmission of visual rays and presenting a black color or a dark color similar to it, and formed into a reflecting face having a flat faced front face 5a and a rear face 5b arranged with many small reflecting faces mutually crossing at 90. when the opening part of a building is set to be a watch section, the light projecting-receiving device 3 is disposed on one side part of the opening part and the reflector 4 on the other side part. When an invader tries to pass through the opening part, near infrared rays R is obstructed by the invader not to be incident to the light receiving element (b), and a detected signal is output from the light receiving part 2 of the light projecting-receiving 3.;COPYRIGHT: (C)2000,JPO
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