An address converter has a base address register and address mask register for setting the start address and range, respectively, of a transparent mode access permitted area. By using the values of these base address register and address mask register, the address converter converts an address signal (address value) supplied from an external bus master into an address signal within a predetermined range from the start position of the transparent mode access permitted area. A transparency controller executes transparent mode access by using the converted address signal. This makes it possible to connect devices having different address bus widths to an external bus without forming any new external circuit, and to set an arbitrary transparent mode access permitted area.