An image forming apparatus 100 includes an intermediate transfer belt 21, a reading section 25, and an intermediate transfer unit 2 having a supplementary roller 26. The intermediate transfer belt 21 is passed over a drive roller 22 and an idle roller 23, and moves along a predetermined circulating path. The reading section 25 reads a concentration of a toner image from a surface of the intermediate transfer belt 21. The supplementary roller 26 is in contact with a rear face of the intermediate transfer belt 21 in such a manner that a gap between a reading face of the reading section 25 and the surface of the intermediate transfer belt 21 is kept constant. The supplementary roller 26 is disposed on a downstream side of the reading section 25 in a horizontal direction by a predetermined distance S. The predetermined distance S is a distance from a center M of the reading section 25 to a point of contact P at which the supplementary roller 26 is in contact with the intermediate transfer belt 21 in a horizontal direction.