At step S1, an analog signal is input. At step S2, a high potential and a low potential input to a DA converter are respectively selected. At step S3, a first AD conversion of the analog signal is performed. At step S4, a voltage range is narrowed down by respectively switching the high potential and the low potential on the basis of conversion data from the first AD conversion. At step S5, a second AD conversion of the analog signal is performed. At step S6, the conversion data from the first AD conversion and conversion data from the second AD conversion are combined to be output as a single digital signal.