Abstract: An improved copy protection circuit of a data stored in a memory capable of advantageously protecting an illegal copying of a data stored in a memory such as a mask ROM (Read Only Memory) by storing a data in a security data store cell. The protection circuit includes a security data store circuit for storing a security data encoded in accordance with a chip enabling signal applied thereto, a switch circuit for outputting a switching signal controlled by the security data, and a data converting circuit for converting an input address by receiving the input address and the security data and for outputting the converted address or the input address to a X, Y-address buffer/decoder in accordance with an output of the switch circuit.