PURPOSE: To enable bar codes with a constantly uniform density to be printed irrespective of exhaustion of an ink ribbon, by a system wherein after printing a bar code, the density of an end part of a printed bar and that of a central part of the bar are read by an optical sensor, and when the difference between the two densities thus read is larger than a preset value, the bar is over printed.

CONSTITUTION: An optical sensor (e.g., CCD sensor) 5 is fitted to a carrier 1, and is capable of reading an image on a paper 4. The density of an upper part (a) of a printed bar and that of a central part (b) of the bar are read by the sensor 5, the difference between the two densities is detected, and when the difference is larger than a preset value, the bar is again printed. By this system, irregularities in the density of the printed bar can be easily corrected.