PROBLEM TO BE SOLVED: To provide a method for checking a shading correction plate in an image reader and for eliminating the influence thereof. SOLUTION: The method includes a step of generating shading correction data from a plurality of positions in the subscanning direction in order to identify defective positions of the shading correction plate and a step of comparing the result of shading correction at a plurality of positions in the subscanning direction with a result of 2nd shading correction at a plurality of the positions in the subscanning direction to identify a defective position on the shading correction plate. A means for comparison detects also such defects where the magnitude of coincidence/dissidence between the comparison results is equal to or greater than a prescribed value. After specifying the defective position, shading correction data are produced from the parts without defects. When there are no parts without defects, positions with least defects are selected or positions at which defects are confined to a prescribed area are selected.
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