PURPOSE: To obtain a stable performance by providing a new case which has pierced plural holes at an infrared ray passing position, on the outside of a case containing an infrared ray emitting device and a photodetector.

CONSTITUTION: An infrared ray emitting device and a photodetector 1 having a lens 2 is contained in a case 3 made of, for instance, a synthetic resin for making an infrared ray transmit. Also, a new case 4 provided so as to cover the case 3 is positioned to the case by a bolt 5. By forming the case to a double structure in this way, a ratio for making the lens 2 contact to the open air becomes small, therefore, it does not occur that the lens is to be white due to frozen with dew. Accordingly, a stable performance is obtained irrespective of a season and a temperature.