An image forming apparatus having an optical scanning apparatus, the optical scanning apparatus comprising: a light source including a plurality of light emitting points (411, 412, 413, 414) each emitting a beam; a rotating polygon mirror for deflecting multiple beams emitted from the plurality of light emitting points, so that the multiple beams scan a photosensitive member provided for the image forming apparatus and to be rotated, exposure positions of the multiple beams deflected by the rotating polygon mirror on the photosensitive member in one scanning period of the multiple beams being different from each other in a rotation direction of the photosensitive member; a lens for passing through the multiple beams deflected by the rotating polygon mirror, the multiple beams which have passed through the lens exposing the photosensitive member; a holding unit configured to hold first data and second data, the first data being used to control light emission timings of the multiple beams from the light source so that exposure start positions of the multiple beams in a scanning direction of the deflected multiple beams on the photosensitive member align each other, and the second data being used to correct magnification difference among images respectively formed by the multiple beams in the scanning direction, the image forming apparatus comprising a control unit configured to control the light emission timings of the multiple beams based on the first data for correcting the exposure start positions and based on the second data for correcting the magnification difference, wherein the first data and the second data are determined by detecting the multiple beams which have passed the lens before the optical scanning apparatus is incorporated in the image forming apparatus. a light source including a plurality of light emitting points (411, 412, 413, 414) each emitting a beam; a rotating polygon mirror for deflecting multiple beams emitted from the plurality of light emitting points, so that the multiple beams scan a photosensitive member provided for the image forming apparatus and to be rotated, exposure positions of the multiple beams deflected by the rotating polygon mirror on the photosensitive member in one scanning period of the multiple beams being different from each other in a rotation direction of the photosensitive member; a lens for passing through the multiple beams deflected by the rotating polygon mirror, the multiple beams which have passed through the lens exposing the photosensitive member; a holding unit configured to hold first data and second data, the first data being used to control light emission timings of the multiple beams from the light source so that exposure start positions of the multiple beams in a scanning direction of the deflected multiple beams on the photosensitive member align each other, and the second data being used to correct magnification difference among images respectively formed by the multiple beams in the scanning direction, the image forming apparatus comprising a control unit configured to control the light emission timings of the multiple beams based on the first data for correcting the exposure start positions and based on the second data for correcting the magnification difference, wherein the first data and the second data are determined by detecting the multiple beams which have passed the lens before the optical scanning apparatus is incorporated in the image forming apparatus.