1. An image processing apparatus comprising: a dark correction unit configured to obtain a dark corrected image by removing a dark image obtained in a state without radiation irradiation from an object image obtained by radiation irradiation; an addition value obtaining unit configured to analyze pixel values included in a plurality of dark corrected images and, based on an analysis result of the pixel values including a negative pixel value, obtain an addition value used to correct the negative pixel value; and an adding unit configured to add the addition value to the pixel values of the dark corrected image.