In a microcomputer 10 which converts an analog signal Vin into digital data and corrects an error of the digital data, a voltage variation ΔVcc is calculated on the basis of a first conversion value obtained by AD-converting a reference voltage Vref by an AD converter 15 in a state that a standard power source voltage including no voltage variation is supplied to the AD converter 15 and a second conversion value obtained by AD-converting the reference voltage Vref by the AD converting section in a state that a power source voltage for the ordinary operation is supplied to the AD converter 15. The digital data produced by the AD converter 15 by AD-converting the analog signal Vin is corrected using the calculated voltage variation ΔVcc.