PURPOSE: To reduce defective readings by associating the width of a boundary display and a range provided with the boundary display with the tilting angle of the read line of a bar code reader and the width of bars composing a bar code that the read line crosses and deciding on the bar code.

CONSTITUTION: The width of the boundary display M provided between two bar codes A and B is assumed to be Bw. Where the tilting angle that can be set over plural bar codes and the minimum and maximum widths of the bars composing the plural bar codes are θ, N and W, respectively, and where Bw< N×tanθ holds, the bar codes and the boundary display are never detected by the same width in a range of θWπ/2. If Bw>W×tanθ holds, the bar codes and the boundary display are never detected by the same width in a range of 0Wθ. Hence all possible straight lines set over plural bar codes in a range of the tilting angle θWπ/2 provide the boundary display in a range lw crossing the width Bw.