1. An inspection apparatus comprising: a receiving unit configured to receive second image data used in preprinting the second image on a sheet, and first image data used in further printing the first image on the preprinted sheet; a composing unit configured to compose reference image data from the received second image data and the received first image data; a reading unit configured to print the first image on the sheet, on which the second image has been printed using the second image data, using the first image data and to read the printed sheet, to obtain read image data; a processing unit configured to carry out smoothing processing using screen information that has been used in preprinting the second image with respect to a second area where, of the read image data, the second image has been preprinted, and configured to carry out smoothing processing using screen information that has been used in printing the first image with respect to a first area where, of the read image data, the first image has been printed, to generate inspection image data; and an inspecting unit configured to inspect the sheet on which both the second image data and the first image data have been printed, by comparing the inspection image data with the reference image data.