A belt unit includes a rotatable endless belt for receiving a toner image thereon or for conveying a transfer material, wherein the endless belt has a smooth-shaped inner peripheral surface, a first reinforcing member, provided on an outer peripheral surface of the endless belt at one end portion with respect to a belt widthwise direction, for reinforcing the endless belt, and a second reinforcing member, provided on the outer peripheral surface of the endless belt at the other end portion. In addition, a driving roller rotationally drives the endless belt while supporting the inner peripheral surface of the endless belt. In the belt widthwise direction, a first length from an inner edge surface of the first reinforcing member to an inner edge surface of the second reinforcing member is smaller than a width of a region in which the driving roller contacts the endless belt, and a second length from an outer edge surface of the first reinforcing member to an outer edge surface of the second reinforcing member is larger than the width of the region in which the driving roller contacts the endless belt.