An image reading device compares pixel values of an image data, which is output when a document is not at a reading position, with a threshold and generates a first dust detection data; compares a image data, which is output while the document is passing the reading position, with a threshold and generates a second dust detection data; when dust is indicated as being present at a matching position in a main scanning direction in both the first dust detection data and the second dust detection data, generates a third dust detection data; and judges that noise is present when the third dust detection data indicates that dust is continuously present for at least a prescribed length in a sub scanning line direction.