PURPOSE: To make it possible to obtain high resolution by using conventional A-D and D-A converters of low resolution and a differential amplifier.

CONSTITUTION: Analog input A is converted into digital value DH by A-D converter 2-1. Simultaneously, digital value DH is converted into analog value AH by D-A converter 2-2. At this time, a difference between input A and analog value AH is within the range of the resolution of converter 2-1. In differential amplifier 2-3, the difference is therefore multiplied by the resolution of the A-D converter. Then, the A-D conversion of difference ASUB obtained in the above-mentioned way is carried out by A-D converter 2-4. Assuming the current digital value to be DL, digital values DH and DL are the same as the output of a conventional parallel A-D converting circuit in terms of each output, so that the resolution obtained will be equal to squared conventional one.