In prefetch caching program implementation for hiding the delay of data access via a network, the operation of the existing applications with the prefetch program must branch, depending on whether data to access is on the cache and it is necessary to add branch processing to the data access process. In the disclosed invention, means for caching data on a device driver level in a client system and means for storing cached data into an external storage unit are provided. Cache control is accomplished by means for sending cache control commands such as "flush the cache" to the client from the outside of the client and means for executing received cache control commands within the client. Data access delay over a network can be hidden by caching.