There is provided an image reading apparatus that is capable increasing the detection accuracy for dust, scratches, dirt, and the like. Sixty-four lines of sampling points on a reference standard white plate are read, shading correction and averaging are carried out the read image data to suppress random noise components in the image data, the data is then stored in a sampling memory, and dust detection is carried out on the data stored in the sampling memory. Alternatively, shading correction and dust detection are carried out on image data that has been subjected to linear interpolation. It is thus possible to increase the accuracy of detection of dust, scratches, dirt, and the like while ensuring that shading correction is performed at high speed.