A reference voltage generating circuit generates a reference voltage Vref. An A/D converting circuit compares an analog input voltage Vin with the reference voltage Vref to convert the analog input voltage Vin to a digital output value Dout. A measured value storing circuit stores a measured value of the reference voltage Vref in advance and outputs the stored measured value. A user of an A/D converter corrects the digital output value Dout from the A/D converting circuit by use of the measured value of the reference voltage Vref outputted from the measured value storing circuit, thereby obtaining a digital value representing the analog input voltage Vin accurately irrespective of the accuracy of the reference voltage Vref.