PURPOSE: To prevent a memory from entering an overloaded state by predicting the number of pages to be used by a job when the job is started and supplementing free pages by a paging-out process when the free pages on a main storage are not enough.

CONSTITUTION: The system is provided with an in-use page quantity generation part 1 which generates the maximum number of pages belong to the jog as the number of in-use pages of the job by a counter, an in-use page quantity storage part 2 which stores the number of in-use pages, job by job, on a storage medium such as a disk as statistical information on the past actuation of the job, an in-use page quantity prediction part 3 which predicts the maximum number of pages, varying during the job execution and belonging to the job, and outputs the number of pages as the predicted number of in-use pages of the job, and a free page supplementation part 4 which performs the paging-out process by the number of pages calculated by subtracting the number of free pages on the main storage from the predicted number of pages of the job. Consequently, even if a request to secure much memory capacity is made instantaneously after the execution of the job, the memory is prevented from entering the overloaded state.