An original is read by an original reading unit configured to read image
data, at least one of the original reading unit and the original is
driven by a driving motor in a sub-scan direction, and, when an amount of
the image data stored by the image data storing unit becomes equal to or
more than a predetermined amount, the rotational speed of the driving
motor is reduced according to driving control data provided for each of
current rotation angles of the driving motor for finally stopping the
driving motor at a predetermined rotation angle, and a speed in a
sub-scan direction at which the original reading unit reads the original
is reduced. Then, main scan line data are thinned out from the image data
according to thin-out data provided for each driving control data, and
image data of the original are restored.