PURPOSE: To utilize the resolution of an A/D converter sufficiently and to improve data precision after digital conversion, by performing time window function processing in the state of an analog signal.

CONSTITUTION: An integrator 4 regards a positive constant voltage e1 as a signal to be integrated and an integrator 5 regards a negative constant voltage e2 as a signal to be integrated. Outputs V4 and V5 of the integrators 4 and 5 are inputted to an analog adder 6, whose output V4+V5 is inputted to an analog multiplier 7. The multiplier 7 multiplies this V4+V5 as a window function by an analog input signal S' to be processed, so the window function processing is carried out on analog basis; when the peak value of the output V0 of the multiplier 7 is matched with the permitted input of the A/D converter 8, its resolution is utilized efficiently.