An image forming apparatus includes a light beam output unit configured to output a light beam, a deflection unit for deflection scanning in a main scanning direction of a photosensitive member by reflecting the light beam from the light beam output unit, a timing information detection unit configured to detect timing information of the deflection scanning by the deflection unit, a calculation unit configured to calculate a correction amount of the main scanning direction for a next scan based on the timing information, a light beam modulation control unit configured to generate a light beam modulation signal based on image data and the correction amount, and a drive unit configured to drive the light beam output unit based on the light beam modulation signal.