An image forming apparatus capable of achieving improved image quality even when image formation is performed using a plurality of types of drive sources different in characteristics. Image forming units for colors form toner images of the respective colors on respective photosensitive drums each of which is driven by a DC motor for rotation. Encoder sensors detect information on the rotational speed of the photosensitive drums. An image forming unit for black forms a black toner image on a photosensitive drum having an outer diameter larger than that of the photosensitive drums for colors, which is driven by a stepper motor for rotation. An intermediate transfer belt transfers toner images formed on the respective photosensitive drums to a sheet. A motor controller controls a drive frequency of the stepper motor based on information on the rotational speed of the photosensitive drum for black.