Abstract: A signal processing circuit processes a signal based on a quantity of light detected by an infrared detector. The signal processing circuit includes an IV conversion circuit, a filter circuit, and a differential converter circuit. The IV conversion circuit converts a signal of a current value provided from the infrared detector to a signal of a voltage value. The filter circuit allows passage of a signal containing a predetermined frequency component among signals converted by the IV conversion circuit. The differential converter circuit generates a reverse-phase signal of a passed signal that has passed through the filter circuit and provides a differential signal including the passed signal and the reverse-phase signal.