An image forming apparatus which makes it possible to grasp such settings of a phase and a main scanning magnification of laser beams as reduce occurrence of moire, by visually checking a plurality of pattern images. Pattern images are formed using a laser beam irradiated from a reference light source and a laser beam irradiated from an adjustment target light source for which each of different phase values is set as a phase setting value. A phase relationship between the phases of the laser beams is adjusted based on the pattern images. Pattern images are formed using a laser beam irradiated from the adjustment target light source for which each of different magnification values is set as a magnification setting value. The magnification of the laser beams is adjusted based on these pattern images.