PURPOSE: To ensure that an image is freely written or deleted on the surface of a sheet without damaging the image by printing the image on the rear of the sheet, if the sheet is transparent or translucent.

CONSTITUTION: A paper type sensor 1 is a photointerrupter consisting of a combination of a light-emitting diode 27 and photosensor 28, and the sensor 1 is provided at a paper cassette mounting part of a device enclosure 31. A transparent or translucent sheet such as OHP sheet 29 is loaded in a paper cassette 30. Light enters the photosensor 28 from a light-emitting diode 27 after being transmitted through the sheet 29 and the type of paper is identified. If even a single translucent paper is present in the paper cassette 30, an image cannot be printed on the rear of the sheet as the light is prevented from reaching the photosensor 28. Therefore, the light-emitting diode 27 and photosensor 28 are provided at a halfway position between the paper cassette 30 and a printing part 32. The transparent or translucent sheet can be used as OHP sheet by printing an image with the right and left reversed on the rear of the sheet.