Abstract: An image reading apparatus corrects a deviation of a reading position of a document to accurately correct a luminance intensity distribution variation. Light is radiated to a shading plate (102) provided at a portion different from a reading position (luminance-intensity-distribution-variation correction reading line 103b) for a document (P) while displacing an illumination unit (101), and shading correction is carried out based on a luminance value of reflection of the light. Light is radiated to the reading position before the document (P) is transported thereto while displacing the illumination unit (101) in the same direction as a displacement direction of the illumination unit (101) at a time of performing shading correction. The luminance value of the reflected light at this time is compared with the luminance value after the shading correction to generate luminance-value reference data before the reading of the document.