This invention provides a data processing apparatus and a data processing method for reducing the amount of image data denoting a pattern drawn on a whiteboard by a pen or the like being moved so as to designate locations in a designated sequence. Specifically, a coordinate transformation circuit transforms first-precision coordinate data within first image data into coordinate data of a second precision lower than the first precision, thereby generating second image data. Given the coordinate data in the second image data, a changing point detection circuit detects the coordinate data that are changed in keeping with the locations being designated movably by the pen on the whiteboard, thereby generating third image data including the detected coordinate data in the designated sequence.