An image forming apparatus includes the following. An image forming unit forms an image. An image magnification changing unit changes a rotating speed of a polygon motor to change magnification of the image. A reference position detecting unit detects a reference position of a predetermined target component in a vertical scanning direction. A target component position calculating unit calculates a position of the target component based on a detecting result and a signal of a predetermined cycle according to the rotating speed of the polygon motor. A storage unit stores a correction table to correct density unevenness. An image data correcting unit obtains correction data based on the position of the target component and corrects the image data. A parameter correcting unit corrects a parameter according to a timing that the rotating speed of the polygon motor is changed.