An image forming apparatus includes an image forming unit forming images on a first sheet and a second sheet having a higher stiffness than the first sheet, a discharge rotary member pair, and a sheet supporting unit to support the first and second sheets discharged through the discharge rotary member pair. In addition, an abutment surface abuts against upstream ends of the first and second sheets supported on the sheet supporting unit, and an elevation unit moves the sheet supporting unit up and down. A control unit, in a state where no sheet is supported on the sheet supporting unit, executes a first discharge mode of controlling the elevation unit to position the sheet supporting unit at a first position before the discharge rotary member pair starts discharging the first sheet, and a second discharge mode of controlling the elevation unit to position the sheet supporting unit at a second position lower than the first position.