Description of a parallel program is facilitated in a cluster system which loosely couples a plurality of server computers each having a cluster file system. In the cluster system, an application program is executed to generate a process. Using the cluster file system, the process maps a file recorded in a shared disk device to a cluster shared memory (distributed shared memory) area virtually provided in an address space arranged on main memory. By doing so, an access to files is processed as an access to main memory.