A system controlling device having a tree structure includes a data controlling unit that functions as a root device. The data controlling unit is connected to a first image processing unit and a second image processing unit by separate high-speed serial buses. The first image processing unit performs image processing on image data scanned by a scanner, while the second image processing unit performs image processing on image data to be output to a printer. A storage unit is independently connected to the data controlling unit and stores therein image data processed by the first image processing unit and the second image processing unit.