An image forming apparatus includes a toner image carrying drum; and an image transfer unit including an endless belt, first and second belt stretching rollers, and an adjusting unit for adjusting belt movement in a widthwise direction by inclining the first roller relative to the second roller. The adjusting unit includes a contacting member provided between the first roller and the drum and contacting with an inner surface of the belt. The contacting member contacts the inner surface of the belt at a position in a drum side across a common internal tangent between the drum and such a portion of a surface of the first roller as is moved closest to the drum by the adjusting unit, the common internal tangent being the one contacting a portion of the drum adjacent to a drum-belt contact position and contacting a portion of the first roller adjacent to a first-roller-belt contact position.