An image forming apparatus includes a photosensitive drum; an image forming device for forming a toner image on the drum; an endless belt; first and second rollers supporting the belt; a first sensor for detecting a widthwise position of the belt; a first displacing device for displacing the first roller in accordance with an output of the first sensor; a second sensor for detecting a widthwise position of the belt; a second displacing device for displacing the second in accordance with an output of the second sensor; an adjuster for adjusting first and second reference positions such that a tangent line direction of the drum at a position at which the drum contacts the belt is parallel with a moving direction the belt.