PURPOSE: To adjust an access timing to a buffer device by a cache synchronizing device, and to attain the efficient write back management of a cache memory.

CONSTITUTION: The number of pages which are being updated and counted at the time of the previous write back is stored in a updating state recorder 12, and it is compared with the number of pages which are being updated and counted at this time write back. When the number of pages which are being updated is increased, the activating cycle of a cache synchronizing device 11 is shortened. Thus, an access interval to a buffer device 3 by the cache synchronizing device 11 for the write back can be dynamically set. Therefore, the load of the buffer device 3 can be prevented from being increased, and a saturation phenomenon can be prevented.