Compressed image data of different resolutions stored in a hard disk drive is divided into blocks of substantially regular sizes. A determination is made as to whether a required block is stored in the main memory at predefined time intervals. If the block is not stored, the block is loaded into the main memory. Subsequently, the loaded compressed image data is referred to so that data for an image of an area required for display or for an image of an area predicted to be required is decoded and stored in a buffer memory. Of the images stored in a buffer area, i.e. a display buffer, the image of a display area is rendered in a frame memory. The display buffer and the decoding buffer are switched depending on the timing of completion of decoding or the amount of change in the display area.