After detecting a speed fluctuation pattern per rotation of a photosensitive drum, a process that forms an image based on image data while causing the photosensitive drum to rotate at a nonstandard linear speed is executed as an image forming process by driving a process driving motor that drives the photosensitive drum to curb a speed fluctuation of the photosensitive drum by using a driving speed fine tuning pattern. Before executing the image forming process, a fine-tuning-pattern correcting process is executed for correcting the driving speed fine tuning pattern of the process driving motor, based on the nonstandard linear speed.