PURPOSE: To produce the key memory system cheaply without lowering the performance thereof by accommodating the reference bit high in access number or the modification bit in the high speed buffer memory.

CONSTITUTION: The page from the external memory device is stored in the main memory portion 6. In the key memory portion 5, the memory protecting key information set correspondingly to the page is stored. This information comprises an access control bit, a fetch protecting bit, a reference bit and a change bit. The key buffer 4 is a memory which has higher speed and a smaller capacity and in which the reference bit relating to recently made access page and a change bit are memorized. In this manner, by accommodating the reference bit and the change bit high in the access number and renewing them, this system of all the key memory system can be formed costly without deteriorating the performance in comparison with the system in which all the key memory system is constructed by the high speed memory.