PURPOSE: To eliminate the response of a filter caused at the switching conversion by outputting selectively an output of an analog-digital converter connected with an amplifier with a switching means.

CONSTITUTION: The 1st code converting section 10 amplifies an analog receiving input signal at its amplifier A1 and converts and outputs it at an analog- digital converter A/D. The 2nd code converting section 11 and the 3rd code converting section 12 are constituted similarly, but their amplifiers A1, A2, A3 have different amplification factor. In switching a changeover switch section 13 at a digital signal processing section 14, since the switching control at a digital signal region is performed, the filter response is not outputted at the gain switch changeover, allowing to keep the continuity of the digital data at the output point.