A high pass filter is disposed to an output part of a gain control amplifier. A gain control signal change detector detects a change of a gain control signal from the gain control amplifier. Upon detection of a change of the gain control signal, a counter and a clock generator generate a cancel pulse which has a certain width, and using the cancel pulse, a first switch cuts an AC signal which is fed to the gain control amplifier. Further, using the same cancel pulse, a second switch reduces the time constant of the high pass filter. Through this operation, a transient response of a DC offset is eliminated.