PURPOSE: To improve reliability for deciding the propriety of a bar code by automatically collating the result of reading digits with the result of decoding the digits of the bar code.

CONSTITUTION: When error is decided by automatic monitor logic, the still picture of the bar code is fixed on a video monitor, correct digits calculated from the preceding data are displayed in the lower step of a bar code printing part by synthesizing pictures and further, the error is informed of an operator by turning a putty light 5 on. The operator recognizes it by the turning-on of the putty light 5 that any abnormality is generated, and the video monitor is visually confirmed. By comparing the still picture with the correct digits displayed in the lower step, the cause of the error is judged. When any abnormality is judged, the operator stops a printer and executes prescribed recovery corresponding to the cause.