To enable satisfactory shade correction processing without expanding memory capacity.

In an image forming device having a shade correction function for applying shade correction to original image data read by a reading part for reading an original by the photoelectric conversion of an optical image provided by optically scanning an original while utilizing white board data provided by optically scanning a white board by the reading part, this device is provided with a deciding means for deciding whether or not the width of the magnifying power of the white board data is to be switched by comparing the white board data provided from the reading part with a prescribed value each time one original is read by the reading part, each time the kind of original to become the read object of the reading part is changed or during the reading operation of the reading part is stopped, and a gain range switching means for switching the width of the magnifying power of the white board data on the basis of the decided result of the deciding means.