1. An inspection apparatus comprising: a receiving unit configured to receive preprint image data preprinted on a sheet and document image data printed on the preprinted sheet; a composing unit configured to compose reference image data from the received preprint image data and the received document image data; a reading unit configured to read the sheet on which both the preprint image data and the document image data has been printed, to obtain read image data; a processing unit configured to carry out predetermined image process on a first and second area corresponding respectively to the document image data and the preprint image data of the read image data, to generate inspection image data; and an inspecting unit configured to inspect the sheet on which both the preprint image data and document image data has been printed, by comparing the inspection image data with the reference image data.