A peak value of readings of a reference plate is calculated by a peak value calculation
means. The peak value is stored by a storage means. An average value of the readings
of the reference plate is calculated by an average value calculation means. Difference
data between the peak value and the average value is calculated by a difference
data calculation means. A variable correction coefficient is set by a correction
coefficient setting means through use of the difference data and the peak value
as shading correction reference data. Readings of an document image is corrected
by a correction means through use of the correction coefficient. Thereby, dispersion
of document image readings in shading correction is decreased.
