In the image processing apparatus according to the present disclosure, a data processing unit derives a density variation component for at least one modulation period from a difference between a reference plate reading value when the illumination is turned off, a reference plate reading value when the illumination is turned on, and black reference data and white reference data in synchronization with modulation of the SS clock, and corrects the image data with the density variation component. The white reference data indicates the light distribution of the illumination in the main scanning direction, and the data processing unit corrects the density variation component on the basis of the difference between the white reference data and the black reference data, and corrects the image data with the corrected density variation component.