PURPOSE: To widen the reference voltage range as an input to an operational amplifier by charging the 1st capacitor with a voltage difference between an input voltage and a reference voltage and charging the 2nd capacitor with an offset voltage of the operational amplifier so as to add a prescribed voltage to each charging voltage.

CONSTITUTION: The operational amplifier 4 of a voltage comparator compares an input signal voltage from a signal input terminal 8 with a reference voltage from the 1st and 2nd reference voltage input terminals 9, 10 and gives an output to an output terminal 7. The comparator is provided with the 1st and 2nd capacitors 11, 12, the 1st group switches 13, 14, 15, 18, 19 controlled with a clock from clock signal input terminals 21, 22 and the 2nd group switches 16, 17, 20 and the 1st and 2nd groups are controlled conversely for on/off-control. Then the capacitor 11 is charged with a difference between a reference voltage VRE1 and the input voltage, the capacitor 12 is charged with an offset voltage of the amplifier 4, a prescribed voltage is added to each charging voltage and then given to a non-inverting input and an inverting input of the amplifier 4 so as to widen the the range of the reference voltage.