PURPOSE: To provide the virtual storage system which is highly flexibly by performing flexible data conversion at the time of a page-in and a page-out process as to a virtual storage system which employs a paging system.

CONSTITUTION: When a memory management device 1 sends a page-in indication to a pager 2, data before conversion which are read in a temporary storage device 5 from a magnetic disk device 6 or paged out and present in the temporary storage device 5 are converted by a data converting device 4 by the indication of the pager 2 and written in a specific page of a main storage device 3. When a page-out indication is made, the data converting device 4 performs data conversion and writes the data in the temporary storage device 5 and the page is released. Information for the switching of the data conversion and the switching of paging operation is stored in, for example, a paging table and on the basis of the table, the data converting process and paging process are carried out.