A light source lights up at luminance associated with supplied electric current. An electric-current supplying section supplies the light source with electric current. A control section controls the electric-current supplying section. A reading section reads at least one sheet of an original document on which light from the light source is irradiated and generates image data. A determining section determines whether a change in the electric current flowing through the light source is large, based on a predetermined criterion, during a period when the reading section reads the at least one sheet of the original document. When the reading section reads the at least one sheet of the original document, the control section increases electric current supplied to the light source if the determining section determines that the change is large relative to electric current supplied to the light source when the determining section determines that the change is small.