PURPOSE: To improve the reliability of converted data as well as the S/N and at the same time to realize a reduction of cost, by performng an automatic gain control in response to the voltage level of an analog input signal.

CONSTITUTION: A level detecting/controlling circuit 18, a variable resistance circuit network 20 and an operational amplifier 21 are added to various types of A/D converters. Thus automatic gain control is performed in accordance with the level of an analog input signal, and an A/D conversion is possible with high accuracy. As a result, the high reliability is maintained for a converted data. AT the same time, the S/N is improved.