An image forming apparatus capable of easily achieving installation of developing units on a developing tray is disclosed. The disclosed image forming apparatus includes a tray section, a plurality of developing units each provided with guide protrusions, and a developing tray including a plurality of supports to support the guide protrusions of the developing units, and a plurality of guides each adapted to guide a corresponding one of the guide protrusions to a corresponding one of the supports. The guides have widths gradually increasing in a direction from a first end of the tray section to a second end of the tray section, respectively.