A construction is achieved at low cost which reduces the loss of gradation in a dark portion of an image that has been subjected to gamma correction in accordance with characteristics of a display. In an image reading apparatus, to make it possible to more finely reproduce the gradation in the dark portion, the number of gradations expressed by an signal obtained by a CCD is reduced by performing non-linear gamma correction using a gamma coefficient below one, and the resulting signal is transferred to a computer serving as a host computer. In the computer, the transferred signal is subjected to gamma correction using a gamma coefficient that is an inverse of the gamma coefficient used in the image reading apparatus, thereby canceling out the influence of the gamma correction performed in the image reading apparatus.