An automatic gain control circuit that controls a gain of a variable gain amplifier that amplifies a main signal, comprising: a peak detector circuit that detects a peak voltage of an output signal from said variable gain amplifier; an average value detection and output amplitude setting circuit that detects an average voltage of the output signals from said variable gain amplifier and adds a voltage of 1/2 of a desired output amplitude of said variable gain amplifier to the average voltage; and an amplifier circuit that amplifies a difference between an output voltage of said peak detector circuit and an output voltage of said average value detection and output amplitude setting circuit and controls the gain of said variable gain amplifier using an amplification result as a gain control signal (GCT, GCC), said peak detector circuit comprising: a first transistor having a base connected to a positive-phase input terminal of the automatic gain control circuit and a collector receiving a power supply voltage; a second transistor having a base connected to a negative-phase input terminal of the automatic gain control circuit and a collector receiving the power supply voltage; a first emitter follower circuit (Q3, I1) having an input connected to emitters of said first transistor and said second transistor and an output connected to an output terminal of said peak detector circuit; and a filter circuit (Ra, C1) having one terminal connected to the emitters of said first transistor and said second transistor, and the other terminal grounded, said filter circuit comprising a series connection of a filter resistor (Ra) and a filter capacitor. a peak detector circuit that detects a peak voltage of an output signal from said variable gain amplifier; an average value detection and output amplitude setting circuit that detects an average voltage of the output signals from said variable gain amplifier and adds a voltage of 1/2 of a desired output amplitude of said variable gain amplifier to the average voltage; and an amplifier circuit that amplifies a difference between an output voltage of said peak detector circuit and an output voltage of said average value detection and output amplitude setting circuit and controls the gain of said variable gain amplifier using an amplification result as a gain control signal (GCT, GCC), said peak detector circuit comprising: a first transistor having a base connected to a positive-phase input terminal of the automatic gain control circuit and a collector receiving a power supply voltage; a second transistor having a base connected to a negative-phase input terminal of the automatic gain control circuit and a collector receiving the power supply voltage; a first emitter follower circuit (Q3, I1) having an input connected to emitters of said first transistor and said second transistor and an output connected to an output terminal of said peak detector circuit; and a filter circuit (Ra, C1) having one terminal connected to the emitters of said first transistor and said second transistor, and the other terminal grounded, said filter circuit comprising a series connection of a filter resistor (Ra) and a filter capacitor. a first transistor having a base connected to a positive-phase input terminal of the automatic gain control circuit and a collector receiving a power supply voltage; a second transistor having a base connected to a negative-phase input terminal of the automatic gain control circuit and a collector receiving the power supply voltage; a first emitter follower circuit (Q3, I1) having an input connected to emitters of said first transistor and said second transistor and an output connected to an output terminal of said peak detector circuit; and a filter circuit (Ra, C1) having one terminal connected to the emitters of said first transistor and said second transistor, and the other terminal grounded, said filter circuit comprising a series connection of a filter resistor (Ra) and a filter capacitor.