PURPOSE: To speed up the system rise operation of a initial data preserving system by storing information to be preserved in a separate area as initial data after designating the information by using a preservation flag and, when the information of a multiplex virtual memory system is outputted from a main memory device, outputting the information while the initial data are left as they are.

CONSTITUTION: When this system is raised, the virtual address and number of pages of segments to be preserved are found by actuating a initial system data preservation control process 4 and causing a preserving segment referring means 41 to refer to a preserving segment registration file 3. Then whether or not the pages exist in a main memory device 2 is checked by actuating a preservation flag setting means 42, fetching the descriptor 71 of the lading page of the preserving segment from a page management table 7, and referring to the descriptor 71, and when the pages are absent, the preservation flag 72 of the descriptor 71 is turned on. When the pages exist, the preservation flag 83 of the auxiliary storing data 82 of a memory management entry 81 is turned on. Thus an initial data storing area is provided separately.