In AD conversion of a voltage, data continuity is ensured between the results of conversion after amplification and of direct conversion without amplification. In AD conversion operation, an analog signal output from a DA converter circuit is directly converted by an AD converter circuit, and the analog signal is converted after amplification with an expected gain of 2n. Based on resultant data, a gain of an amplifier circuit and an offset thereof are calculated. An analog signal to be enhanced in bit precision is amplified by the amplifier circuit and converted by the AD converter circuit, the offset is subtracted from the resultant conversion, and the result is multiplied by a ratio of the expected gain to the calculated gain to cancel gain error. Based on data with gain error canceled, acquisition of bit-extended conversion result data is performed to ensure continuity between data having different degrees of bit precision.