The A/D converter has first and second PPDC circuits (pulse-phase-difference coding circuits). The first PPDC circuit performs A/D conversions on the reference voltage and on the voltage signal amplified by an amplifier in an alternating sequence, the amplifier using the reference voltage as a potential base thereof. The second PPDC circuit performs A/D conversions always on the reference voltage. The A/D-converted data set of the voltage signal outputted from the first PPDC circuit is corrected depending on the difference between the A/D-converted data set of the reference voltage outputted from the second PPDC circuit when the first PPDC circuit A/D-converts the reference voltage and the A/D-converted data set of the reference voltage outputted from the second PPDC circuit when the first PPDC circuit A/D-converts the voltage signal.