Abstract: A method for fabricating a mask ROM capable of reducing the processing time taken in fabricating the MROM by the manufacturer after the order by the user, thus improving the productivity. For fabricating the mask ROM, first, a fabrication of a MOS transistor is achieved by forming a gate oxide film, a gate electrode and source/drain regions on a semiconductor substrate. After the formation of the MOS transistor, a BPSG film, a metal electrode, a passivation film and a pad are sequentially formed. Finally the implantation of ROM code ions in accordance with the user's order is carried out, completing the fabrication of the MROM. It is possible to reduce the processing time (turn around time) taken in fabricating the MROM by the manufacturer after the order by the user because the ROM code ion implantation is carried out at the final stage of the MROM fabrication.