An A/D conversion unit includes an A/D converter, an analog offset canceller connected to the input end of the A/D converter, and a digital offset canceller connected to the output end of the A/D converter. The analog offset canceller performs a coarse adjustment of offset canceling voltage in the first stage of offset cancellation operation to largely cancel out the offset voltage involved in the A/D conversion unit by shifting the range of the input signal. Then the digital offset canceller performs a fine adjustment of offset canceling voltage in the second stage of offset cancellation operation to completely cancel out a residual offset voltage. Through the offset cancellation operation, the linearity error of the A/D converter, linearity error of the D/A converter, and matching error between the A/D and D/A converters are completely cancelled out.