The optical scanning device includes a storage unit configured to store information on a scanning time of laser light and an emitted position of the laser light in a sub-scanning direction, the laser light measured on a plane. A scanning time for forming one scanning line is set based on the information on the measured scanning time and the measured emitted position, which is stored in the storage unit, and a radius of a photosensitive drum. This allows a magnification error in a main scanning direction to be reduced with an inexpensive configuration while realizing downsizing of an apparatus.