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.