PURPOSE: To improve the stability of an AGC operation by controlling the response speed of an AGC amplifier circuit according to the frequency of a reading signal.

CONSTITUTION: The main body of a variable amplifier 30 is the differential amplifier consisting of a pair of transistors 31, 32 and a pair of the differential signals of a reading signal R0 is respectively inputted to the bases of these TRs. A pair of the differential signals of the reading signal R1 is led out of the collectors connected to a power source via load resistors 31a, 32b respectively. The emitters of the differential transistors TRs 31, 32 are connected to each other via a coupling resistor 33 and are connected via control transistors 34, 35 and emitter resistors 34a, 35a to the grounding point. The amplification rate control signal CS is applied to the common connection base of the control TRs 34, 35, by which the currents of the control TRs are controlled, by which the amplification factors of the differential TRs 31, 32 are controlled.