An image reading apparatus includes an imaging unit which has a plurality of channels and which, for the channels, outputs effective signals output from photo-receiving sections for receiving light from a subject, and optical black signals output from light-shielding sections in which the light from the subject is shielded. The image reading apparatus also includes a monitoring unit for monitoring the optical black signal output from the imaging unit, a correcting unit for performing shading correction on the signals output from the imaging unit, and a controller for controlling timing with which the shading correction by the correcting unit is performed. In an image reading method for the apparatus, the shading correction is performed in response to a job start signal, and based on an optical black signal monitored thereafter, the shading correction is performed again.