PURPOSE: To easily diagnose which is defective, a control part or a printing mechanism, or easily perform the analysis of inferiority, in performing diagnosis, by comparing the output of a control part for controlling printing operation with that of a diagnosis control part for controlling the printing operation at the time of diagnosis.

CONSTITUTION: At the time of the diagnosis of a printer, a diagnosis control part 3 for controlling a printing mechanism part 21 to perform printing operation is provided and the output of said diagnosis control part 3 is compared with that of a control part 20 for controlling printing operation at the time of usual operation to perform the diagnosis of the printer 2. By this method, when the comparing result of a comparing part 4 does not coincide with each other, the defective of the control comparing result is fed back to a higher rank apparatus 1 and, therefore, the analysis of the deflective thereof can be performed. When a defective is generated in printing operation even through the comparing result of the comparing part 4 coincides, the defective of the printing mechanism part 21 can be detected.