To realize an image reader which improves productivity in image reading without deteriorating image quality.

An optical system unit (5 to 12) scans an original placed on the glass 1 of an original platen, an original image is photoelectrically converted into an electrical signal, and shading member 15 is used to apply shading correction to the electrical signal. The shading member 15 is arranged at a necessary minimum position (B) to accelerate the optical system unit to a position which is located between an original image reading start position (E) and a stop position (HP) of the optical system unit and at which the optical system unit gains a prescribed constant scanning speed that corresponds to an operation mode.