An image forming apparatus includes an image carrier, an image forming means for forming a patch image on the image carrier, a light emitting means, a plurality of light receiving means adjacently arranged so as to receive light reflected from the patch image when light is irradiated by the light emitting means onto the patch image which moves with movement of the image carrier and each including one or more light receiving elements, and an output means for outputting an output signal that depends on a difference between a received light quantity of a first light receiving means and a received light quantity of a second light receiving means that are respectively odd-numbered and even-numbered in the arrangement order of the light receiving means.