Abstract: An automatic gain control system for controlling a gain of an A/D converter automatically by controlling reference voltages of said A/D converter at the time of analog-to-digital conversion of a signal which signal being clamped by a clamp circuit to a clamp level which is set to a center level before entering said A/D converter, comprising a clamp level control circuit for detecting the clamp level from an output of said A/D converter to control the clamp level; a first A/D converter for digital-to-analog converting an output of said clamp level control circuit; an automatic clamp level control system.