An image forming apparatus includes a transfer belt having a light penetrating portion formed at one side of an edge portion of the transfer belt, a protection tape adhered to both sides of the edge portion to cover the light penetrating portion, and a guide member attached to one side of the protection tape. A plurality of rotation rollers drive the transfer belt in a caterpillar-type manner and include a guide groove formed at both ends of each rotation roller such that the guide member is inserted therein. A sensing body receives one end of the transfer belt to be positioned therein. A first sensing element and a second sensing element are disposed at both sides of the transfer belt such that a second distance from a transfer surface of the transfer belt to the second sensing element is longer than a first distance from the other surface of the transfer belt to the first sensing element.