In an image forming apparatus, an image forming portion forms an image on a rotator. A storage portion stores change characteristics information relevant to correction parameters corresponding to phase points of the rotator. A designating portion designates a correction parameter from the correction parameters based on the change characteristics information. A correcting portion corrects an image forming position on the rotator based on the correction parameter designated by the designating portion. When a first determining portion determines, based on a detecting phase point of the rotator detected by a detecting portion, that the current phase of the rotator corresponds to a shifting phase point, and further a second determining portion determines that the image forming portion is in inactive time, the designation by said designating portion is shifted to the correction parameter corresponding to the shifting phase point.