A magnification correction controller corrects magnification in a main scanning direction by changing a timing of an image signal based on a time difference between a first detection time in a predetermined position along a main scanning line of the image carrier to which the laser beam is irradiated and a second detection time in a position different from the predetermined position along the main scanning line. The magnification correction controller corrects the magnification in the main scanning direction by changing a timing of the image signal corresponding to a color different from a predetermined color, based on the time difference measured for the predetermined color.