PURPOSE: To decode bar code without making a conversion rule complicated by rearranging the information of the bar code through conversion processing or preprocessing of conversion.

CONSTITUTION: To extract bar code data to be converted actually from the obtained data of the bar code, the head blank (left margin) and the tail blank (right margin) are detected and the bar code is decoded as to the data between the two detected data. Namely, the signal detected by a detecting means is rearranged according to the widths (thickness) of white bars and black bars constituting a start code, bar code characters and an end code on the basis of the standards of the bar code to judge that the bar code is large or small, and respective records are determined according to their module constitution while matching with the modules of the start code, end code, and characters. Conversion into a bar code is started unless the maximum width value is ≥1.5 time as large as the minimum width value.