1. A distributed file system comprising: a storage device for holding files; multiple clients for carrying out file operations on said storage device; a server using tokens to control rights to file reading and writing operations by the clients; and a network coupling said clients, said storage device and said server, wherein said server contains a token revoke request means for sending a token revoke request for demanding a return of a token granting rights to write on said file, to a first client that holds said token, wherein said token revoke request means sends said token revoke request, which includes information identifying a second client that requested said file, and information indicating a level of said token requested by said second client, said level being either read or write, wherein said first client comprises a memory section for holding file data loaded from said storage device and a data output means for sending said file held in said memory section and relating to said token, to said server for said second client that requested said token when said token revoke request is received, wherein said data output means sends data in a range among files linked by said token, to said server for said second client that requested said token, and performs synchronous processing on said storage device by writing data in a range among said files not linked by said token, and wherein said data output means decides whether to send said token of said file held in said memory section to said server for the second client that requested the token, or to write said file in said storage device and perform synchronous processing on said storage device, based on an input/output capacity of said network and said storage device and a data size of said file sent to said server for said second client that requested the token.