An image reading device feeds a document original along a sub-scanning direction over an exposure glass. A moving unit moves to and fro in the sub-scanning direction over the exposure glass thereby causing an image reading position on the exposure glass to move. A line image sensor reads one line of an image of the document original at the image reading position in a main scanning direction, while the moving unit is moving, thereby obtaining image. A dirt determining unit determines that there is dirt on the exposure glass when a straight line appears in the main scanning direction in the image data.