An image forming apparatus that is capable of scanning at a proper speed at all positions on a main scan line without being affected by variations in the characteristics of a f-θ lens, variations in the laser beam wavelength, irregularities in the rotational speed of a polygon mirror and fluctuations in the characteristics of the f-θ lens due to changes in ambient temperature. An image clock signal for exposure control is generated for each of a plurality of segments obtained by dividing a main scan line on a photosensitive drum scanned by a laser beam, based on a modulation coefficient. The modulation coefficient is corrected based on the detected phase difference between detection timing of the terminating end of the main scan line and generation timing of the last image clock signal for the last segment.