PURPOSE: To eliminate the need for increase in the dynamic range of an operational amplifier by fetching an input signal in a short time interval to detect the level difference, using a level difference to apply interpolation integration thereby eliminating a switching noise in the input signal.

CONSTITUTION: An input signal Vin is sampled at a sample-and-hold circuit 1 by using sampling clocks s1, s2 and held. The signal sampled by the circuit 1 is subject to operation of level difference ΔV by a difference detection circuit 2. Then an interpolation integration circuit 3 uses the level difference ΔV to apply interpolation integration and its output signal is fed to an A/D conversion circuit 4. The level difference of input signals at two points is detected to apply A/D conversion, then switching noise by sampling switch is cancelled to apply highly accurate A/D conversion.