An image forming apparatus that is operable at a plurality of image forming speeds and can be constructed at low costs without adding circuits or the like. A conversion circuit converts image data into lighting patterns for turning on/off laser light on a basis of each of units of auxiliary pixels formed by dividing a pixel as an image forming element. A shift register sequentially stores lighting patterns for pixels pixel from the conversion circuit, and sequentially outputs them to a laser drive circuit. In monochrome printing, the rotational speed of a polygon motor is set to perform a printing operation. In color printing, the difference between an image forming speed for the monochrome printing and an image forming speed for the color printing is calculated, and based on the difference, the amount of insertion of pixel pieces using the shift register is determined for a printing operation.