An image forming apparatus includes a photosensitive member and a scanning unit including a light source, a rotatable polygonal mirror, and a sensor. The image forming apparatus includes setting of an operation in a first mode and setting of an operation in a second mode. The image forming apparatus further comprises, a surface identifying portion and a correction data storing portion configured to prestore correction data including first correction data for a first rotational speed and second correction data for a second rotational speed. Positional deviation in a main scan direction of laser light is corrected on the basis of the first correction data or the second correction data.