An image forming apparatus that is capable of carrying out image formation on a recording medium such as plain paper without increasing the FCOT (First Copy Out Time) and is also capable of carrying out optimal image formation on a recording medium, such as thick paper, for which the processing speed is reduced with no registration misalignment between the leading ends of toner images and the leading end of the recording medium. An image is primarily transferred onto a rotatively driven image carrier, and the image on the image carrier is secondarily transferred onto a recording medium. An image writing reference position signal for starting image formation is issued based on the circumference of the image carrier which is the length of the image carrier in the direction of rotation thereof or based on a detected reference position on the image carrier. A user can selectively switch, through operation of an operating section, between the issuing of the image writing reference position signal based on the circumference of the image carrier and the issuing of the image writing reference position signal based on the detected reference position.