PURPOSE: To make it possible to store effectively a large quantity of signal in a small-capacity storage device, by subjecting an original signal to be stored to the delta modulation and storing one sample data with a one-bit recording capacity.

CONSTITUTION: In formation of an approximate output, +1 or -1 is added to preceding data when data y(tn) is 1 or 0, and a step size Δ is applied. An input signal f(tn) is compared with an approximate output x(tn-1), and the bit sequence of the output y(tn) of the comparator is binary 0 or 1, and 1 is generated in case of f(tn)>x(tn-1), and 0 is generated in other cases. Thus, since the signal is so stored that one data corresponds to one bit, the capacity of the storage device is reduced considerably, and an integration circuit is omitted.