To provide an infrared detector with high accuracy and high sensitiveness capable of achieving a simple circuit structure by a constitution wherein an infrared ray detecting circuit is formed such that four thermally sensitive resistors having the identical resistance-temperature characteristics are connected to an operational amplifier.

An infrared detector is equipped with an infrared ray detecting circuit that comprises resistors R1, R2, R3, R4 having the identical resistance- temperature characteristics and an operational amplifier 2. An applying terminal for a bias voltage Vb is commonly connected to each one end of each of the resistors R1, R4. The resistor R3 is serially connected to the other end of the resistor R4. A connection point between the resistor R3 and the other end of the resistor R4 is connected to the inversion input terminal of the operational amplifier 2 and the other end of the resistor R3 is connected to the output terminal of the operational amplifier 2. One end of the resistor R2 is serially connected to the resistor R1 and the connection point to the other end of the resistor R1 is connected to the non-inversion input terminal of the operational amplifier 2. A fixed voltage Vx to be applied to the other end of the resistor R2 is set by a resistor R5 and a variable resistor VR1.