An image forming apparatus includes a plurality of image bearing members configured to bear respective images, an optical writing unit configured to write the respective images on the plurality of image bearing members by deflecting a laser beam on a polygon mirror, a transfer unit configured to transfer the respective images onto an image receiving member, and a controlling unit configured to perform an adjustment of a difference in phase of respective rotation speeds between the plurality of image bearing members at a timing of one of non-image forming operations during a series of image forming operations performed by the plurality of image bearing members, the optical writing unit, and the transfer unit.