To reduce detecting means and to simplify a structure of a thermal transfer printer by constituting a means for detecting conveyance of a recording sheet to a predetermined position and a means for detecting mounting of a ribbon cassette of one detecting means.

A sheet detecting photosensor 17a of a sheet detecting sensor for detecting conveyance of a recording sheet P is provided at a position in which the sheet P on a sheet guide 3 is passed, and a reflecting plate 18a is arranged at a ribbon cassette 4 side opposed thereto. Accordingly, when the cassette 4 is mounted in a printer body 1, the photosensor 17a indicates an ON state, and recognizes mounting of the cassette 4. When a leading end of the sheet P arrives between the photosensor 17a and the plate 18a, the photosensor 17a indicates an OFF state, and recognizes conveyance of the sheet P to a predetermined position.