To provide a sheet-width detector that detects the width of each of sheet-like mediums in a wide range of size from small width to large width and allows downsizing.

The width of a sheet mounted on a sheet feeding tray is detected by rotating a rotary variable resistor 15 using a plurality of gears along with the movement of a width restricting unit 20 disposed on the movable side of a sheet-width restricting member 3.