A computer system includes a main memory, a lower class memory, and a secondary storage medium and executes an operating system, an in-memory computing program, and a prefetch optimizer program. The in-memory computing program writes processing target data including a plurality of data objects stored in the secondary storage medium into a plurality of continuous areas on a virtual memory space and executes a process while accessing the continuous area. When detecting that the operating system executes a class-in process triggered upon a page fault for a predetermined virtual page, the prefetch optimizer program acquires information of the continuous area from the in-memory computing program and directs the operating system to execute a class-in process for virtual pages included in the predetermined continuous area including the predetermined virtual page.