An image forming apparatus which is capable of reducing first print output time and preventing slack of an intermediate transfer belt in a monochrome mode. Surfaces of photosensitive drums are electrically charged and exposed to light, thereby allowing electrostatic latent images to be formed thereon. Developers are attached to the electrostatic latent images to form visible images transferred to the intermediate transfer belt. Among the photosensitive drums, a photosensitive drum used in the monochrome mode and the intermediate transfer belt are rotatably driven by a first rotatably driving unit, and photosensitive drums other than the photosensitive drum used in the monochrome mode are driven by a second rotatably driving unit. In the monochrome mode, the rotational speed of the second rotatably driving unit is controlled so as to be equal to or less than that of the first rotatably driving unit without adjusting rotation phases of the photosensitive drums.