An image forming apparatus which forms an image by scanning a light beam in a main-scan direction of a photosensitive member, the apparatus including: an output unit which irradiates a light beam for exposing the photosensitive member; a deflection unit which deflects a light beam from the output unit to scan the photosensitive member in the main-scan direction; a light-receiving unit which detects the light beam deflected by the deflection unit; a computation unit which calculates difference information by using information of a detected time; a control unit which drives the deflection unit by setting a parameter for controlling operation of the deflection unit; a prediction unit which predicts a correction amount of a scan width in the main-scan direction in a next scan; a modulation unit which generates a driving signal for driving the output unit; and a driving unit which drives the output unit.