A printer includes platen opening/closing means for increasing a gap between a platen and a print head when a printing medium is set to the printer, and cleaning means for cleaning a photo sensor device. The cleaning means removes paper dust, for example, accumulated to such an extent as to interrupt the optical path between the light emitting and receiving elements of the photo sensor device. The cleaning means is controlled by a control means contained in the printer. The control means drives the cleaning means when an output signal level of the photo sensor device is below a predetermined signal level after the printing medium is discharged out of the printer.