To maintain constant detection accuracy while reducing the installation space for an optical sensor that detects the density of toner of a test patch. An image forming apparatus comprises: a roller that is arranged between a drive roller and a driven roller at a position on the downstream in the circulation direction with respect to the drive roller while pressing an outer surface of an intermediate transfer belt; and a density measuring part that is arranged between the drive roller and roller and optically detects the density of a test patch TP that is formed at a predetermined position in the width direction on the outer surface of the intermediate transfer belt. The density measuring part includes: an optical sensor that is arranged between the drive roller and roller above the outer surface of the intermediate transfer belt for measuring a position facing a passage route of the test patch TP in the width direction; and a support member that is in contact with an inner surface of the intermediate transfer belt at a position facing the optical sensor in the circulation direction across the intermediate transfer belt.