There is provided a multi-screen synthesis apparatus that can execute display of video data and update of an OSD image without causing a user to feel a visual sense of incongruity, and reduce system costs. Periodic video source data and aperiodic OSD image data are written into a unified memory reserved for planes. The video source data and the OSD image data are read from the unified memory, based on a synthesis layout, for simultaneous display on a single display in a synthesized state. Video data to be written into the unified memory is decimated in units of a frame on an input video source-by-input video source basis. The decimation of the data is controlled based on display priority of the video data determined based on a multi-screen display layout.