An image forming apparatus includes a plurality of image forming units for forming an image on a printing medium such as a sheet of paper. A plurality of color types of toner are arranged on a transport path of the printing medium. Toner images corresponding to the color types of toner are color-by-color transferred onto the printing medium. Light emitted from a flash lamp melts and fixes the toner images. The fixing order of the toner images corresponding to the color types of toner is decided so that the toner images are fixed in ascending order of toner's absorptivity of the light emitted from the flash lamp or in descending order of toner's reflectivity of the light.