FIG. 2 shows a memory protection system according to an embodiment of the present invention. In FIG. 2, reference numeral 1 designates a table which stores memory protection information provided in correspondence to the respective memory areas, and numeral 2 identifies a register which stores memory protection information corresponding to respective programs to be executed by the processor. Now, explanation will be made of the memory protection information employed in the present embodiment. Let us assume that the utilization of the memory as shown in FIG. 1 is employed. Then, a peculiar memory key is allotted to each of the subsystems a, b, c and so on. For example, the memory keys "0", "1" and "2" are assigned to the subsystems a, b and c respectively. For programs within each subsystem, it may be established that a control program can access the areas P.sub.1, P.sub.2 and P.sub.3 and a user program can access only the areas P.sub.2 and P.sub.3. In the present example, therefore, memory keys "0", "1" and "2" are given as protection information to the areas P.sub.1, P.sub.2 and P.sub.3, respectively, while program keys "0" and "1" are given to the control program and the user program respectively. A program key indicating one subsystem is also provided for each program. By allowing access of a program to the memory when the program key in that program indicating a certain memory subsystem coincides with the memory key allotted to the subsystem and the key given to the program for areas in the subsystem is smaller than or equal to the key given to the memory for areas in the subsystem, the above-required memory protection can be realized.