An image forming apparatus comprising: an elevating plate provided in a cassette so as to be able to ascend in a range from a bottom position to a top position with sheets of recording medium placed thereon; a roller group (62, 64p, 64q) at least including a pickup roller to take up and feed the top of the sheets of recording medium placed on the elevating plate during a printing process; a print unit that forms an image on the recording medium fed by the roller group (62, 64p, 64q) during the printing process; a drive mechanism at least including a motor, the drive mechanism raising the elevating plate by means of a drive force from the motor until the top of the sheets of recording medium placed on the elevating plate contacts the pickup roller; and a slit plate having a plurality of slits provided therein, the slit plate being movable in an amount proportional to an amount of movement of the elevating plate while the elevating plate ascends from the bottom position to the top position, 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 in a blocked state, characterized in that: the slit plate is adapted such that any two neighboring slits from among all of the slits have different width ratios; and the image forming apparatus further comprises: a control unit having memorized therein positions of the elevating plate that correspond to ratios of neighboring-slit widths, the control unit identifying a position of the elevating plate that corresponds to a ratio of neighboring-slit widths obtained on the basis of widths of neighboring slits derived from a detection signal outputted by the sensor unit when the motor is driven at a constant rotational speed, thereby deriving a remaining quantity of sheets on the elevating plate. an elevating plate provided in a cassette so as to be able to ascend in a range from a bottom position to a top position with sheets of recording medium placed thereon; a roller group (62, 64p, 64q) at least including a pickup roller to take up and feed the top of the sheets of recording medium placed on the elevating plate during a printing process; a print unit that forms an image on the recording medium fed by the roller group (62, 64p, 64q) during the printing process; a drive mechanism at least including a motor, the drive mechanism raising the elevating plate by means of a drive force from the motor until the top of the sheets of recording medium placed on the elevating plate contacts the pickup roller; and a slit plate having a plurality of slits provided therein, the slit plate being movable in an amount proportional to an amount of movement of the elevating plate while the elevating plate ascends from the bottom position to the top position, 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 in a blocked state, characterized in that: the slit plate is adapted such that any two neighboring slits from among all of the slits have different width ratios; and the image forming apparatus further comprises: a control unit having memorized therein positions of the elevating plate that correspond to ratios of neighboring-slit widths, the control unit identifying a position of the elevating plate that corresponds to a ratio of neighboring-slit widths obtained on the basis of widths of neighboring slits derived from a detection signal outputted by the sensor unit when the motor is driven at a constant rotational speed, thereby deriving a remaining quantity of sheets on the elevating plate. the slit plate is adapted such that any two neighboring slits from among all of the slits have different width ratios; and the image forming apparatus further comprises: a control unit having memorized therein positions of the elevating plate that correspond to ratios of neighboring-slit widths, the control unit identifying a position of the elevating plate that corresponds to a ratio of neighboring-slit widths obtained on the basis of widths of neighboring slits derived from a detection signal outputted by the sensor unit when the motor is driven at a constant rotational speed, thereby deriving a remaining quantity of sheets on the elevating plate. a control unit having memorized therein positions of the elevating plate that correspond to ratios of neighboring-slit widths, the control unit identifying a position of the elevating plate that corresponds to a ratio of neighboring-slit widths obtained on the basis of widths of neighboring slits derived from a detection signal outputted by the sensor unit when the motor is driven at a constant rotational speed, thereby deriving a remaining quantity of sheets on the elevating plate.