PURPOSE: To obtain a square wave signal which rises and trails correspondingly to a bar code, by subjecting a photo-electric conversion signal to primary and secondary differentiations and combining these primary and secondary differentiated signals to obtain the peak point of the primary differentiated signal.

CONSTITUTION: Photo-electric conversion signal (b) obtained by scanning bar code (a) is differentiated to obtain primary differentiated signal (c). This signal (c) generates a peak position at the inflection point in the half-tone section of leading and trailing parts of signal (b). Next, signal (c) is further differentiated to obtain secondary differentiated signal (d). Next, the positive part of signal (c) and the negative part of signal (d) are subjected to AND operation to obtain the positive peak point of signal (c), namely, leading of square signal (e). Meanwhile, the negative part of signal (c) and the positive part of signal (d) are subjected to AND operation to obtain the negative peak point of signal (c), namely, trailing of square signal (e). This signal (e) is an accurate square wave signal corresponding to the bar code.