PURPOSE: To make the differentiation of the components of an extremely low region extractable and to make high speed and high accuracy computation possible by digItizing signals abd obtaining differentiated signals through arithmetic processing not containing multiplication and division.

CONSTITUTION: A measuring analog signal X is converted to a digital signal X0 by an A/D converter 1. The differentiation of the low region components out of this signal X0 is accomplished only by the addition and shift operation with a digital arithmetic device 3, and is again converted 2 to an analog signal which is amplified 4, whereby a differentiated signal Z is outputted.