To judge whether an ink ribbon cassette for performing a double-side printing is erroneously mounted or not in the case of mounting the cassette even in the case of removing the cassette from a printer on the way of using.

(a) In the case of printing in a traveling direction of an arrow by a print head 19 in the state the an ink ribbon cassette is mounted in a printer by using its A-surface, a mark detecting sensor 20 detects a Y-mark 51b next to a Y-mark 51a, and the printer stores that the A-surface is being used. In the case of removing and remounting the cassette from and at the printer, (b) at the time of mounting in the state of using a B-surface, when an ink ribbon 51 is wound in a traveling direction of an arrow, the sensor 20 detects the Y-mark 51a next to the Y-mark 51b. Accordingly, the printer stops printing by judging a B-surface mounting irrespective of using of the A-surface.