An image forming apparatus having: a slit plate in which a plurality of slits of different widths are provided, the slit plate being movable in an amount proportional to an amount of movement of a elevating plate; a sensor unit disposed such that the slits pass between a luminous element and a light-sensitive element, the sensor unit outputting a detection signal that indicates whether light emitted by the luminous element is in a transmitted state or not; and a control unit that stores positions of the elevating plate corresponding to the widths of the slits, and identifies a position of the elevating plate corresponding to a slit width derived from a detection signal outputted by the sensor unit, thereby deriving a remaining quantity of sheets on the elevating plate.