An image reading device of the present invention reads a reference white plate before an actual document image in order to generate shading correction data. As for the first line, an input data determining circuit unconditionally determines that input data is normal. A sum data selecting circuit selects input data if the input data is normal or selects a value stored in a white data storage if otherwise. A sum data calculating means forcibly makes data output from white sum data storage (logical) ZERO, i.e., black data and then writes the result of calculation in the above storage. As for the second and successive lines, a decision value calculating circuit calculates a noise decision value for each pixel by using the white data of a previous line stored in the white data storage. The input data determining means compares the noise decision value and input image data to thereby determine whether or not the input image data is normal.