An image forming apparatus includes an image forming unit which transfers an image obtained through exposure and development to paper, and a control unit which changes a rotation speed of a polygon mirror at an intermediate area on a carrier to change a reduction/magnification ratio of an image. The control unit changes the rotation speed in a stepwise manner in such a way as to allow a stable rotation of the polygon mirror in each step, and controls the image forming unit to form a corrected patch image in parallel with the stepwise change of the rotation speed. The corrected patch image is obtained through correction in accordance with the stepwise change of the rotation speed to be the same as the patch image formed when the rotation speed is not changed.