PURPOSE: To provide an infrared detector in which only signal light is detected and outputted without using a circuit for removing a background infrared component.

CONSTITUTION: Two pairs of opposed electrodes 3a, 3b and 3c, 3d with each opposed direction perpendicular each other are formed on a P-type narrow band gap semiconductor layer 2. A magnetic field is applied normal to a semiconductor substrate 1, and a constant current is provided to a pair of the opposed electrodes 3a and 3b, and an infrared detector is maintained to a temperature in which the potential difference generated to a pair of the opposed electrodes 3c and 3d is zero when background infrared rays are made incident. Accordingly, when signal light is inputted without removing the background infrared rays on a signal processing circuit (extraction of the signal light), the infrared detector has an effect of generating an output voltage corresponding to the signal light.