An image forming apparatus includes an image forming mechanism for forming an image, at least one guide member arranged in parallel, and a belt unit to be inserted into and removed from the image forming apparatus. The belt unit includes a belt, at least two rotating supports, a supporting frame, a cover, and at least one guided member. The belt is formed in an endless belt shape and transfers the image formed by the image forming mechanism. The belt is looped over the rotating supports which are rotatably supported by the supporting frame. The cover covers an outer circumferential surface of the belt. The guided member is disposed on an outer surface of the cover, engages with the guide member, and is slidably moved along the guide member when the belt unit is inserted into and removed from the image forming apparatus.