PURPOSE: To facilitate detection of the bar code and simplify the device, by setting a black bar code and a white bar code as one pair and expressing this pair with a ratio of width in respect to the bar code pattern for information read printed on a paper or the like.

CONSTITUTION: A bar code 5 consisting of a pair of a black bar code and a white bar code has a narrow black width and a wide white width, and a code 6 has a wide black width and a narrow white width. Codes 5 and 6 are expressed with, for example, 0 and 1 respectively. In case of reading of bar codes from the left, if rates of the black width and the white width of the code 5 are compared with each other just after the start of the code 6, the bar code 5 is judged to be 1 or 0. It is sufficient if the read speed is constant approximately only while the reader passes through the code 5, and the difference between rates of the narrow width and the wide width has a margin and is detected easily even if the read speed is not uniform. Thus, the device is simplified, and bar codes are detected even if their precision is low in a certain degree.