To provide an image reading apparatus capable of suppressing deterioration in the productivity of the apparatus when an abnormality occurs by enabling the apparatus to perform only monochromatic reading if only the adjustment of G output is normally completed even if any one or both of R, B outputs are out of an adjustment gain range and generate errors.

The image reading apparatus uses a photoelectric conversion element 9 having a plurality of deposition colors (R, G, B) for converting a reflection light into electric signals for reading an original 14, reads a reference white plate 15 thereof, and performs gain adjustment so that the level of read digital image data thereof reaches a predetermined level before image reading starts after power source is turned on. In this apparatus, error processing when a result thereof is out of the set gain adjustment range is constituted so that full-color reading after that is stopped and only the monochromatic reading may be available if the G signal is not an error and both or any one of the R, B signals are errors.