An image forming apparatus comprising: a photosensitive member; a beam scanning device including: a plurality of light sources configured to emit light beams for exposing the photosensitive member, a rotary polygon mirror configured to deflect a plurality of light beams emitted from the plurality of light sources such that the plurality of light beams scan the photosensitive member, an optical member (204, 207) configured to guide the plurality of light beams deflected by the rotary polygon mirror to the photosensitive member, a storage unit configured to store first correction data for correcting a light amount of the light beam, the first correction data including a plurality of data items associated with the plurality of light sources, respectively, the first correction data being correction data for correcting changes of the light amount of each of the light beams in a scanning direction in which the light beam scans the photosensitive member, caused by the optical characteristics of the optical member (204, 207); an output unit configured to output second correction data for correcting the light amount of the light beam to the beam scanning device associated with non-uniformity in potential characteristics of each of a plurality of areas on a surface of the photosensitive member, the second correction data being correction data for being commonly used in correction of the light amount of the light beam emitted from each of the plurality of light sources; and a control unit provided in the beam scanning device and configured to control the light amount of the light beam to be emitted from each of the plurality of light sources, based on the first correction data including the plurality of data items associated with the plurality of light sources, respectively, which is stored in the storage unit, and the second correction data input from the output unit. a photosensitive member; a beam scanning device including: a plurality of light sources configured to emit light beams for exposing the photosensitive member, a rotary polygon mirror configured to deflect a plurality of light beams emitted from the plurality of light sources such that the plurality of light beams scan the photosensitive member, an optical member (204, 207) configured to guide the plurality of light beams deflected by the rotary polygon mirror to the photosensitive member, a storage unit configured to store first correction data for correcting a light amount of the light beam, the first correction data including a plurality of data items associated with the plurality of light sources, respectively, the first correction data being correction data for correcting changes of the light amount of each of the light beams in a scanning direction in which the light beam scans the photosensitive member, caused by the optical characteristics of the optical member (204, 207); an output unit configured to output second correction data for correcting the light amount of the light beam to the beam scanning device associated with non-uniformity in potential characteristics of each of a plurality of areas on a surface of the photosensitive member, the second correction data being correction data for being commonly used in correction of the light amount of the light beam emitted from each of the plurality of light sources; and a control unit provided in the beam scanning device and configured to control the light amount of the light beam to be emitted from each of the plurality of light sources, based on the first correction data including the plurality of data items associated with the plurality of light sources, respectively, which is stored in the storage unit, and the second correction data input from the output unit. a plurality of light sources configured to emit light beams for exposing the photosensitive member, a rotary polygon mirror configured to deflect a plurality of light beams emitted from the plurality of light sources such that the plurality of light beams scan the photosensitive member, an optical member (204, 207) configured to guide the plurality of light beams deflected by the rotary polygon mirror to the photosensitive member, a storage unit configured to store first correction data for correcting a light amount of the light beam, the first correction data including a plurality of data items associated with the plurality of light sources, respectively, the first correction data being correction data for correcting changes of the light amount of each of the light beams in a scanning direction in which the light beam scans the photosensitive member, caused by the optical characteristics of the optical member (204, 207); an output unit configured to output second correction data for correcting the light amount of the light beam to the beam scanning device associated with non-uniformity in potential characteristics of each of a plurality of areas on a surface of the photosensitive member, the second correction data being correction data for being commonly used in correction of the light amount of the light beam emitted from each of the plurality of light sources; and a control unit provided in the beam scanning device and configured to control the light amount of the light beam to be emitted from each of the plurality of light sources, based on the first correction data including the plurality of data items associated with the plurality of light sources, respectively, which is stored in the storage unit, and the second correction data input from the output unit.