Image reading apparatus and image reading method

An image reading apparatus comprising:

• a guide plate for guiding an original document placed at a document reading position;
• a transportation part for transporting the original document in such a manner that the original document can be guided by said guide plate to pass through the document reading position;
• a reading glass arranged at the document reading position in opposition to said guide plate;
• a shading plate arranged at a shading plate reading position;
• a scanning carriage arranged to freely reciprocate in such a manner that it is able to read, through said reading glass, said guide plate or the original document that passes through said guide plate at the document reading position, as well as to read said shading plate at the shading plate read position;
• and
• a control part for controlling said scanning carriage in such a manner that said scanning carriage is moved to the shading plate reading position to read said shading plate, performing shading processing, moving said scanning carriage to the document reading position, reading said guide plate through said reading glass, comparing pixel data obtained when reading said guide plate with pixel data obtained when reading said shading plate, thereby to determine whether there is the presence or absence of abnormal densities in pixel data obtained when reading said guide plate, and determining that reading positions at which the pixel data of abnormal densities have been detected are abnormality occurrence reading positions when it is determined that there is the presence of abnormal densities.