An image reading device includes a reading unit for reading a document. The reading unit includes red color green color and blue color light sources and a plurality of light receiving elements aligned in a main scanning direction. In a pre-scan performed precedent to a main-scan, the reading unit reads a part of the document with a lower reading resolution than the reading resolution in the main-scan in order to gather information about the document placed on a document reading table. An optimum light amount to be emanated from the light source at the time of main-scan is set while changing the light amount from the light source and checking the outputs from the light receiving elements. An optimum light amount to be emanated from the light source at the time of pre-scan is calculated based on the optimum light amount set for the main-scan.