An image processing apparatus for obtaining image information by detecting, as an image signal, an electrical signal corresponding to a reflectance or transmittance of an original, including: an A/D converting circuit for converting an analog signal outputted from a photoelectric conversion element into an n-bit digital signal; and a shading correcting circuit for correcting an output characteristic of the photoelectric conversion element, high m bits of the n-bit digital signal being set as effective bits when white shading data is obtained, low m bits of the n-bit digital signal being set as effective bits when black shading data is obtained, bits being added as low bits to the m-bit white shading data, bits being added as high bits to the black shading data, shading calculation being performed. In the image processing apparatus, a capacity of a shading memory can be suppressed without reduction in quantifying resolution of the black shading data.