An image forming apparatus improved in the accuracy of detecting a time period over which a laser beam is scanned. A laser beam emitted from a laser light source is deflected by a polygon mirror such that the laser beam scans a photosensitive drum. The deflected laser beam is guided onto the photosensitive drum via an imaging lens. A mode hop detection unit detects a mode hop of the laser beam. An image pulse generation section detects a main scanning magnification of an electrostatic latent image formed on the photosensitive drum. When a mode hop is detected, the main scanning magnification in a next scanning cycle is corrected using a correction value stored in a storage unit and used most lately, whereas when no wavelength variation has not detected, the main scanning magnification in the next scanning cycle is corrected using the latest correction value.