PURPOSE: To prevent the lack and position deviation of an output image caused by a sheet size setting error, or a jam during conveyance.

CONSTITUTION: A slit 1a is provided along the paper feed direction A of a paper supply tray 1, and resistors 2a, 2b are formed at the back face on both sides of the slit 1a. Piercing the slit 1a, a sheet guide 3 and a slider 4 are integrated and mounted slidably to the back face of the paper supply tray 1, and the resistors 2a, 2b are short-circuited by a movable plate spring 5 fitted to the slider 4. It is so formed that one ends of the resistors 2a, 2b can be connected to the connecting terminals 7a, 7b of a device body side tray receiving part 6, and the sheet guide 3 is brought into contact with one side end of stacked sheets to enable the detection of sheet size as resistance value information.