PURPOSE: To obtain an AGC detecting circuit having short attach time and long release time by constituting the circuit by transistors (TR), capacitors, a differential amplifier and an output circuit having different charging and discharge time.

CONSTITUTION: The circuit consists of a TR4 whose base is connected to a bias circuit comprising resistors 2, 3 and a signal source 1, a capacitor 17 whose one end is connected to the emitter of the TR4 and whose other end is grounded, a differetial amplifier comprising a TR6 whose emitter is connected to a constant current source 12 and a TR7 whose emitter is connected to the TR6 and whose base is impressed with a DC bias by a bias power supply 13, and the output circuit having different charge/discharge time to a capacitor 16 connected between an output terminal 15 and grounded GND and whose input terminal is connected to the output terminal of the differential amplifier. The output circuit consists of TRs8, 9 and resistors 10, 11, 14.