A file server achieving sufficient power-saving effect is provided. The file server is capable of operating an on-line storage medium (in a state in which reading/writing can be started immediately in response to a file read/write request) and an off-line storage medium (which has to be started up upon receiving a read/write request). Upon reception of a stream file write request, the file server's processing unit acquires bit rate information from the stream file, calculates the file's splitting position from the bit rate information and startup time of the off-line storage medium, and stores a first part (up to the splitting position) in the on-line storage medium while storing the remaining second part in the off-line storage medium. Upon reception of a stream file read request, the processing unit reads out the first part while starting up the off-line storage medium and reading out the second part.