PURPOSE: To hold the secrecy of information by writing and reading address information or data in or out of an ROM through a multiplexer whose state is set according to each cipher signal during the writing and reading.

CONSTITUTION: A user supplies an in-writing cipher signal input 13 to a cipher contents initializing circuit 14 in advance to place the multiplexer 15 in a correspondig state. The X address in the address signal from an X address recorder 5 is passed through the multiplexer 15 to access an ROM storage area 9. At this time, address modification is specified by the multiplexer 15 set in the state corresponding to the cipher signal and write data is written in the modified address of the ROM storage area 9. Thus, the secrecy of the information is kept.