PURPOSE: To realize the hierarchy structure of programs and the separation of memory access ranges, and to achieve careful memory protection, by giving different priority levels to the programs.

CONSTITUTION: Priority information 4 outputted from a memory key register 1 is compared by a comparator 10 with priority information 8 from a protect key register 7, and section information 5 is compared by a comparator 11 with section information 9. A discriminating circuit 14 for discriminating whether access is allowed or not receives a priority information compaison signal 12, a section information comparison signal 13, and access limit information 6, and outputs a discrimination signal 15. An access error reporting circuit 16, once receiving a memory access signal 17, sends an interruption request signal 18 to a processor when the discrimination signal 15 shows an access error, and a write request signal 19 to a memory is inhibited by an NAND circuit 24.