1. An image scanning apparatus comprising: a memory for storing shading data acquired when manufacturing the image scanning apparatus; an image processing unit for comparing the shading data acquired when manufacturing the image scanning apparatus with shading data acquired when using the image scanning apparatus and, based on the result of the comparison, processing an image of an original documents, wherein the image processing unit determines whether a comparative value of the shading data acquired when manufacturing the image scanning apparatus and the shading data acquired when using the image scanning apparatus is within a predetermined critical value range and processes pixels as failed pixels when the comparative value of each of the pixels is not within the predetermined critical value range, and wherein the image processing unit calculates a standard average shading value by dividing an average of the shading data of pixels not including the failed pixels, acquired when using the image scanning apparatus, by an average of the shading data of the pixels not including the failed pixels, acquired when manufacturing the image scanning apparatus, obtains a value for each of the failed pixels by multiplying the standard average shading value by the shading data of the failed pixels acquired when manufacturing the image scanning apparatus, and sets the value as new shading data of the failed pixels.