reading apparatus which can reduce a change of color balance due to the exchange of the optical unit the image reading apparatus comprises an original glass plate on which an original is placed and an optical unit adapted to move in parallel to the original glass plate below the original glass plate the optical unit comprising an illumination light source adapted to illuminate the original placed on the original glass plate and a charge coupled device adapted to read the reflected light from the original placed on the original glass plate and the optical unit being exchangeable with a spare optical unit the image reading apparatus retains thickness information of the original glass tableplate the spare optical unit retains change information of an image resolution of the image reading apparatus due to a change of an optical path length of the spare optical unit replacing the optical unit application_number US 4758108 A description preferred embodiments of the present invention will be now described in detail with reference to the drawings fig1 is a view schematically illustrating an arrangement of an image reading apparatus according to an embodiment of the present invention in fig1 this image reading apparatus comprises an original glass plate 1 on which an original 2 is placed a shaft 7 disposed below the original glass plate 1 in parallel thereto and directed in a sub scanning direction and an optical unit a which moves on this shaft 7 the optical unit a comprises an illumination light source 3 which illuminates the original placed on the original glass plate 1 a ccd sensor 4 and an imaging lens 5 which images the reflected light from the original placed on the original glass plate 1 on the ccd sensor 4 the ccd sensor 4 reads an image which is.