To reduce the malfunction of an infrared detector even when high-frequency electric field noise enters the detector by grounding transparent conductive films and a metallic cap while the films and cap are electrically connected to each other.

A pyroelectric element 1 is mounted on a printed wiring board 2 with supporting bodies 61 and 62 in between and circuit parts 21 and 22, such as the FET, resistor, etc., constituting a circuit are fitted to the rear surface of the board 2. An optical filter 5 forms a light ray entrance window 41 and a frequency-selective insulating film 52 at the light ray entrance window section of an infrared detector sealed with a metallic cap 4. In the detector, at least one main surface of the filter 5 is coated with transparent conductive films 71 and 72, electrically connected to the cap 4, and grounded. Even in the part of the filter 5, therefore, high-frequency electric field noise which hinders the detection of infrared rays is grounded through the films 71 and 72 and cap 4.