An access system for accessing a storage area network (SAN) has a shared disk device provided on the SAN, and a client for accessing a file in a file system created in the shared disk device. In the client, a functionality of accessing a file system in the shared disk device is installed as part of a user program of the client by using a dynamic link library. The client preferably has access processing means for inserting, as the functionality of accessing, access processing for accessing the file system in the shared disk device into ordinary file access processing. The access processing means being installed as a dynamic link library.