PURPOSE: To eliminate easily a power supply frequency noise superimposed on an input signal, by sampling the input signal at plural number of points from a zero cross point of a power supply freuency with an equal interval and at a posivie and a negative phase.

CONSTITUTION: Plural analog input signals applied to terminals 11∼1N of an input switching device 1 are switched and selected with switches S1∼SN amplified at a preamplifier 2 and inputted to a sample-and-hold circuit 3, where the signal is sampled and held. An output of the circuit 3 is A/D-converted in high speed at an A/D converter 4 in the form of sequential comparison, and its digital signal is inputted to an operating circuit 5. A signal generator 6 generates a signal corresponding to a zero cross point of a frequency of an AC power supply AC with a control signal from the circuit 5. The signal is inputted to a timing circuit 7, which applies a timing signal to the switching device 1, the circuit 3 and the converter 4. The circuit 3 samples the signal at plural points from the zero cross point of the power supply frequency with an equal interval and at a positive and a negative phase and the circuit 5 operates a mean value at each point of a negative and a positive phase with an equal position from the zero cross point and gives an output.