The image forming apparatus determines, based on a determination value calculated by a calculating unit, a light amount of reflection light from a first position on an image bearing member from light amounts of reflection light beams from a second position and a third position on the image bearing member where a patch image is not formed in a state where the patch image is formed at the first position on the image bearing member, or determines the light amount of reflection light from the first position on the image bearing member by detecting the light amount of reflection light from the first position on the image bearing member with a detecting unit in a state where the patch image is not formed at the first position on the image bearing member.