PURPOSE: To ensure printing on a blank having a specified size by comparing the size of a cassette attached to a printer controlling section with a specified one and informing discrepancy between them if, any, while providing a means for stopping printing operation.

CONSTITUTION: In a paper supply operation, a cassette size designating switch 15 on a control panel is first operated so that either a cassette 2 or 3 is designated and stored. Also, then, switches 16W19 are turned on/off by positions of magnets provided on the backs of attached cassettes 2, 3 so that the cassette size is judged in a microcomputer 23 through a switch condition detecting circuit 25 according to the turned on/off condition of the switches to be stored in a memory 24. Next, the attached cassette size is compared with the designated one and if the discrepancy between them is found, a buzzer or the like is operated while the printing operation is controllably stopped.