PURPOSE: To quickly carry out reproduction at an appropriate level without affecting an external constitution and to cope with the fluctuation of an input by normally slowing down AGC's responsiveness so as to sufficiently reproduce a low frequency and temporarily speeding up the responsiveness only if an input voltage suddenly changes.

CONSTITUTION: A difference voltage detection circuit 8 doubles as a secondary filter 6, and a switch 10 is formed with diodes D1 and D2. It is assumed that where the normal direction voltage of the diodes D1 and D2 and their normal direction resistance are VF and RD, respectively, the difference V between a 1st order voltage V1 and a 2nd order voltage V2 exceeds the voltage VF. Then the diode D1 or D2 is conducted. If the voltage V1 suddenly rises to attain V1 - VF>V2', the diode D1 is conducted, and a capacitor C2 is charged by a time constant C2.RD. Since, normally the normal direction resistance RD is several tens of Ω, RD.C2 becomes dominant over the time constant of a 2nd order filter 6 if a resistance R2 is set several KΩ. The time constant RD.C2 is far smaller than the time constant C1.R1 of a 1st order filter 4. Conversely, if the AGC corresponds to a bottom part B, the diode D2 is conducted to attain a low time constant(quick responsiveness) similarily.