To provide an infrared radiation detecting device at a low coat by solving a problem that coat reduction and miniaturization are impossible because an impedance conversion circuit of the conventional infrared radiation is composed of a high resistor, a junction FET and a load resistor.

A pyroelectric element 1 is connected between the non-inverted input terminal of an operational amplifier OP1 and the reference potential of the operational amplifier OP1. The inverted input terminal of the operational amplifier OP1 is connected with the output terminal of the operational amplifier OP1. An output terminal is formed from the output terminal of the operational amplifier OP1 via an amplifying circuit 2. By this constitution, the number of components can be reduced.