A stored data modifier modifies data starting at any arbitrary address on a storage medium that should output a data word with a multi-byte width. A masked ROM outputs a 2^N-byte data word starting at an address specified as a multiple of 2^N by an address signal. A correspondence detector determines whether or not correspondence is found between a correction address and one of a number 2^N of addresses starting at, or preceding, the address specified by the address signal. If the correspondence detector has found the correspondence, a stored data selecting section selectively outputs, on a byte-by-byte basis, either the output of the masked ROM or correction data in accordance with the address signal and the correction address.