PROBLEM TO BE SOLVED: To provide an image reader that can detect dust with high accuracy. SOLUTION: A shading correction value is generated, on the basis of read data along the main scanning direction at a first position of a lightness reference board and stored in a line memory 502. Then by using the shading correction value, shading correction is conducted (using a multiplier 503) to read data along the main scanning direction at a second position of the lightness reference board different from the first position and the obtained data, after the correction are stored in a line memory 101. Then the data after the correction are read from the line memory 101 and analyzed by each pixel. As a result of the analysis, when e.g., a defective pixel whose data value differs from that of peripheral pixels is found, it is discriminated that the 1st position of the lightness reference board has a lightness fault and the second position of the lightness reference board has no lightness abnormality.