PURPOSE: To eliminate an I/O bottleneck of a library backing store stored with a PLM when the same PLM in simultaneous multiple use is executed.

CONSTITUTION: While the PLM 1032 is loaded, a page to be paged out which is selected under LRU control, etc., is paged out of a main storage 1050 to the page backing store 1081 even if the object page is a page which is not altered such as a code/constant page 1082. When the page is accessed, a missing page processing means 1070 for missing page exceptional generation time pages the page in the main storage 1050 from the paging backing store 1081. Consequently, when the same PLM in simultaneous multiple use is executed, the I/O bottleneck of the library backing store stored with the PLM is eliminated to prevent the system and jobs from deteriorating in performance.