When an optical image obtained by an optical scanning unit of optically scanning an original sheet automatically fed by an original sheet feeder is read, it is discriminated in units of pixel whether or not dirt or scratch attached to a running reading glass plate is present. Moreover, the number of the dirt and the width of the dirt are detected, and an image reading position which does not come under the influence of the dirt as much as possible is set based on the detected number of the dirt and the detected width of the dirt even when the dirt is detected at all the reading positions.