To provide a comparator which is realizable by a simple circuit structure, which can prevent the incorrect detection of a peak without necessity of a full wave rectifying circuit and which can accelerate and to provide an AGC circuit.

A positive phase input signal is inputted to the gate of a first transistor PT101, an opposite phase input signal is inputted into the gate terminal of a second transistor PT102, and the drains of the transistors PT101, PT102 are connected to the low impedance inputs ND102 of a current mirror circuit CURM. Differential reference potential is applied between the gate terminals of third and fourth transistors PT103, PT104, and the drains of the transistors PT103, PT104 are connected to the high impedance output terminal ND103. The direction and the amplitude of the current signal outputted from the high impedance output terminal to the external circuit are changed according to the magnitude relation between the differential input signal magnitude and the differential reference potential.