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