Density unevenness caused by an optical facet angle error or the like of a polygon mirror which has a plurality of reflection planes is corrected after shipment with a simple configuration. At least one of the plurality of reflection planes is identified as a reference plane. A plurality of pieces of light amount adjustment data for changing the light amount of light deflected for each reflection plane with the reference plane as a reference, and at least one piece of light amount correction data for correcting the light amount based on the plurality of pieces of light amount adjustment data are stored. When a light source emits light, one mode can be selected out of a test print mode (S5) in which light is emitted in a light amount based on the plurality of pieces of light amount adjustment data and a corrected print mode (S13) in which light based on the light amount correction data is emitted.