There is provided an image forming apparatus which is capable of reducing a period of time required for obtaining an image output, in the case of changing the image forming operation from a monochromatic image formation to a color image formation during execution of the image forming operation. A plurality of image forming units form images and overlap images the formed images onto a transfer material. A preparation for image formation is started in a first mode in which image formation is carried out by the plurality of image forming units while image formation is being carried out in a second mode in which image formation is carried out by at least one of the image forming units. The image formation is carried out in the first mode after the image formation in the second mode is completed.