An image forming apparatus includes a photosensitive medium on which an electrostatic latent image is formed, an exposure unit to irradiate light onto the photosensitive medium, a developing device to apply toner to the photosensitive medium having the electrostatic latent image formed thereon, a cleaning device to remove waste toner remaining in the photosensitive medium, and a waste toner collecting device to store the waste toner removed from the photosensitive medium. The waste toner collecting device is provided with a supporting rail to support the developing device. The waste toner collecting device includes the supporting rail that supports the developing device, so that additional components to support the developing device are not needed, and the whole size of the image forming apparatus is reduced.