PURPOSE: To secure the contents of memory circuit of an electronic computer by supplying an exclusive OR output between the write data and the set code to the memory circuit then reading an exclusive OR between the read data of the memory circuit and the set code.

CONSTITUTION: An encoded code "1010" set to the outside of a memory is set to a register 3 by a code setting circuit 4. While data "1101" is set to a write data receiving signal 1 by a write command given from a CPU. Then an exclusive OR circuit 5 obtains an exclusive OR between the data "1101" and the code "1010" of the register 3. Thus "0111" is obtained and stored 6. In a read mode, the circuit 5 obtains an exclusive OR between the data "0111" of the memory circuit 6 and the code "1010" to obtain "1101" which is transmitted through a read data transmitting circuit 2.