PURPOSE: To accurately judge the logic of a bar code.

CONSTITUTION: A circular bar code plate 13 in which specific information for a film cartridge is written is rotated to be interlocked with a spool rotary shaft, a wide black and white pattern expressing a logical '1' and a narrow one expressing a logical '0' are alternately arranged on the bar code plate 13, so that a pattern width is detected as a signal width in accordance with the level change of an outputted signal caused by reflectance on the black and white patterns by using a reflection type photosensor. In the case of judging the logic based on the signal width; a unit width is calculated based on the signal width previously detected just before, and the signal width is compared with a doubled calculated unit width; the logical '1' is stored as data if the compared signal width is large, which means the wide pattern, and the logical '0' is stored as the data if the compared signal width is small, which means the narrow pattern; and the logic is judged while updating reference judged one after another. Thus, the logic is accurately judged without being affected by rotating irregularity.