A differential infrared detector has: a detection element which detects infrared radiation in a predetermined detection area, converts a radiation energy thereof into an electric signal, and outputs the signal; and a discrimination circuit which calculates a variation amount of the radiation energy, based on the output signal, and generates a human body detection signal if the variation amount is equal to or greater than a predetermined level. This differential infrared detector comprises: an ambient temperature detection circuit which generates a voltage, based on an ambient temperature in the detection area; and a sensitivity correction circuit which is arranged to increase detection sensitivity of the output signal produced by the detection element if the ambient temperature is within the predetermined temperature range, based on the voltage.