To provide the mutual backup method for a plurality of servers by which the client server system with high reliability is realized at a low cost.

When a client requests update of data to a server A (access server), the server A inquires about a time stamp stored in all other servers B, C. Since the time stamp of the server A is To a time stamp reply from the servers R, C to the server A is To, the server A confirms it that data stored in the server A are not old. Moreover, a time T1 at this point of time is stored in the server A as the time stamp for updated data this time, and the server A makes data update request this time to the other servers B, C and distributes the time stamp T1 to them. The other servers B, C update the data and the time stamp similarly in the case of the server A. The server A receives data update reply from the other servers B, C and the data update reply is returned to the client.