An image reading device capable of preventing, by increasing the quantity of light at the center or bound portion of a spread book document, the quantity of light from becoming short and preventing density from becoming irregular due to the deviation of a focus. At and around the center of a spread book document where the document is bound, the quantity of light for illumination or the amplification gain of image data read from the document is increased. A plurality of fluorescent lamps illuminate the surface of the document at the upstream side and downstream side with respect to an intended scanning direction. When the edge of the document is to be detected, only one of the lamps is turned on to enhance the stripe pattern representative of the stepped edge portion of the document.