A situation where, even though a file system set with an additional function has been expanded, the additional function is unable to operate is prevented from occurring. Provided are a file control computer and its system having a main computer including a file system for managing reading and writing of data between the client computer and the storage device, a control circuit for controlling the file system, and a memory circuit for storing management information of the file system. The memory circuit stores setting information which is set with an additional function that is set to the file system, and a condition necessary for operating the additional function. Upon expanding the file system based on the management information, the control circuit determines a status of fulfillment of the condition in the expanded file system, and outputs warning information if the determination result denies fulfillment of the condition.