PURPOSE: To prevent the influence of the fluctuation of an offset voltage source, by separating the reference potential of a D/A converter in an A/D converter from the ground and connecting one end of a signal to be converted to this reference potential.

CONSTITUTION: An input analog signal 10 is applied across a reference terminal (-) 120 and a reference terminal (+) 130 and is converted to a digital signal by an A/D converter 100 and is outputted from an output bus 150. The reference terminal (-) 120 is in the low potential side of a resistance row of a D/A converter, which is provided in the A/D converter 100, and is separated from a ground terminal 140. The output of a reference power source 30 is applied to the high potential side of the resistance row of the D/A converter. Consequently, even if an offset voltage source 20 is fluctuated, this fluctuation has no influence upon the digital value on the output bus.