PURPOSE: To obtain a correct digital data corresponding to a substantial waveform by providing a detecting and holding circuit for a peak value of a read input signal from an optical card, an output means and a updating means.

CONSTITUTION: The device is provided with a photoelectric signal converter 1, a peak value detecting holding circuit 2, an offset circuit 3, a comparator 4, a monostable multivibrators 5 and 6. Then the peak value of a read input signal from an optical card is held by the peak value detecting holding circuit 2 and the output of a digital data is outputted after holding, and the peak value is updated after said output. Thus, the output of a correct digital data corresponding to a substantial waveform is obtained.