PURPOSE: To automatically perform the offset correction at a digital signal level by extracting sign bits while subtracting an offset correction value from an input digital signal obtained after A/D converting an analog signal and outputting a carry when continuously detecting the identical code.

CONSTITUTION: When the output digital signal of the identical polarity is continued over the period corresponding to more than 1/2 of the maximum cycle of the input signal receiving the band limit, an offset detection means 4 transmits the carry after deciding the period is non-input period without input signals and the offset exists. A correction value storage means 5 starts the setting of the offset correction value, and the carry is counted up or down by an addition means for each prescribed cycle during the identical polarity is continued in the output digital signal. Since it indicates the correction value reaches a proper amount when the polarity of the output signal is different from the former sampled value, the increase and decrease of calculated values is stopped, this proper amount of correction value is subtracted from the digital input signal after that, and the offset correction is performed.