PURPOSE: To enable the function of a sensor to be sufficiently maintained even in long-time continued use, by providing a cassette-mounting means with a sensor for detecting a ribbon cassette, and mounting a means to be detected by the sensor onto the ribbon cassette correspondingly to the sensor.

CONSTITUTION: With a ribbon cassette 1 mounted on a cassette support base 10, lock parts 10a, 10b of the support base 10 are engaged with engaging cutaway parts 2a, 2b of the cassette 1, whereby the cassette 1 is fastened to the support base 10. As a result, a metallic plate 8 is pressed against contacts 11a, 11b, so that the contacts 11a, 11b are electrically connected to each other, and a cassette-detecting sensor 20 detects the metallic plate 8 as a means to be detected by the sensor. The sensor 20 outputs a signal to a controlling part, which judges that 'the ribbon cassette is present'. Thus, mounting of the cassette can be surely detected and speedily, and because there is no movable part, higher durability can be contrived.