PURPOSE: To attain reading of an original in matching with the fluctuation of luminous amount by providing a white reference plane to the outside of a sub- scanning start edge on an original platen glass and utilizing the reflected light from the white reference plane ahead the start of scanning so as to correct properly the luminous amount distribution.

CONSTITUTION: The white reference plane 9 is fixed to the outside of the sub- scanning start edge in the same plane of an original platen on the original platen glass 1. When a read start signal is inputted an optical scanning unit 8 starts movement, the white reference plane 9 on the original platen glass 1 is scanned at first, the reflected light is made incident to a read element 7, the luminous intensity level is converted into an electric signal and stored in a RAM as the white reference level of the original. Then the level outputted at each scanning of the original from the RAM is used as a white reference level and a read signal is obtained. Thus, even if the deterioration due to secular change of the light source 4 exists, an accurate original read signal is obtained.