An image forming apparatus including a main body provided with an opening, a cover to open and close the opening, a plurality of development units developing electrostatic latent images into visible images through developers, a transfer device including an intermediate transfer belt receiving the developers from the plurality of development units and transferring the developers to printing media, and a sensor unit provided with light windows located at first positions opposite the intermediate transfer belt when the opening is closed by the cover, and located at second positions exposed to the opening when the opening is opened by the cover. Thereby, a user may clean the light windows through the opening.