Paper feed tray unit for a printer

A paper feed tray unit for a printer comprises: a paper sensor for detecting a paper on a support; a guide position sensor formed of a reflection photosensor for detecting the position of slide guides; and a paper width detection sheet with marker lines having reflectances corresponding to paper sizes. A controller detects a paper on the support using the paper sensor. If no paper is present, the controller recognizes the position of the slide guides based on a signal from the guide position sensor detected from the paper width detection sheet, and controls a drive mechanism to move the slide guides to a position corresponding to a paper size input by a user before placing a paper on the support. This enables easy paper setting on the support, and prevents the slide guides from moving if a paper is present on the support, preventing the paper from being crushed.