A technology capable of preventing generation of a problem due to saturation of an output value in an image pick-up section caused by a variation of illumination light from a light source. An image reading device includes a plurality of light sources each of which illuminates a document surface as a target for reading with illuminating light of a color different from the other light sources, an image pick-up section for imaging light radiated from the plurality of the light sources and reflected on the document surface, a controlling unit for making the plurality of the light sources emit light with any one of the plurality of the light sources as a reference light source, and an adjusting unit for lowering a current value of a power source which makes the reference light source emit light when an amount of light imaged in the image pick-up section exceeds a threshold value as a predetermined upper limit value.