A non-volatile IC memory comprising in addition to a PROM region divided into blocks a RAM region having a capacity corresponding to one of the blocks wherein the entire data stored in the corresponding block in the PROM region designated by an address sent out from the outside is transferred to the RAM region. After data in the corresponding portion in the RAM region designated by the address is rewritten by data sent out from the outside, the data in the RAM region is written back in the corresponding block of the PROM, thereby data is rewritten by a word unit or a bit unit.