PURPOSE: To accelerate a swap processing by limiting the object of swap-out to a physical page, which is required for swapping-in a process, only out of all the physical pages used by the process.

CONSTITUTION: A process swap-out means SF3 swaps out only the physical page, which is required for swapping-in the process selected by a swap-in process selecting means SF1, out of the physical pages belonging to the process, which is selected by a swap-out processing selecting means SF2, according to a page swap-out means SOF. The passing time of a process control table is updated to '0' corresponding to the swapped-out process and process state information is updated to 'OUT' to show a swapped-out state, respectively. Then, the number of the swapped-out pages is added to a swap-out page number. Thus, the swap processing is accelerated.