An input information obtaining portion of a control section obtains requests input from an input device by a user, which requests include a display region moving request to enlarge/reduce or scroll an image displayed on a display device and a request to generate/erase a viewport, change the size of a viewport, or move a viewport. A viewport control portion successively determines the number, arrangement, and size of viewports accordingly. A display region determining portion determines the region of an image to be displayed next in each viewport. A loading portion determines tile images to be newly loaded, and loads the data of the tile images from a hard disk drive. A decoding portion decodes the data of tile images used for rendering the image in each viewport. A display image processing portion updates the display region independently for each viewport.