To provide a seal for preventing an ink ribbon cassette from being set inside out in a carriage and a cassette applied with such a seal.

Marks 3, 4, 5, 6 are put on corresponding parts of a seal and an ink ribbon cassette so that the seal can be applied easily to a predetermined position of the ink ribbon cassette. In case of a double sided ink ribbon cassette, the seal is peeled off upon finishing use of one side and then the ink ribbon cassette is turned down so that the seal can be applied again to the same position on the rear side.