PURPOSE: To improve variation characteristics of a source voltage and to perform high-accuracy comparing operation even with small hold capacity by providing a compensating means for an input offset voltage and a compensating means for a step error due to the opening and closing of a switch.

CONSTITUTION: Switches 1 and 3 are closed and a switch 2 is open throughout the sampling period of a 1st input signal VA1. When the switch 3 is open, charges Q0 held in a capacity element 5 is Q0=C{VA1-(VB+V10)}. Here, V10 is the input offset voltage of an amplifier 7 and VB is the voltage of a constant voltage source 8. Then when the switch 1 is open and the switch 2 is closed, a 2nd input signal VA2 is applied to the capacity element 5 and the offset voltage is compensated to accurately compares the VA1 and VA2 with each other. Further, a capacity element 6 having the same capacity value that the capacity element 5 has is added to cause the held charges to vary finely in phase with a differential input, so the step error can be compensated.