To provide a network data backup system which can prevent the loss of data due to an unexpected accident and a disaster.

The system is provided with a means storing data, when data is stored in one server, also in other two or more servers, a means with which a plurality of servers holding same data mutually monitor a fault and a means with which the normal server holding data similar to data which the broken server holds cooperatively searches the server having unused data capacity, decides an alternate server being the alternative of the broken server from the servers having unused capacity, transfers data to the alternate server and stores it. The system is constituted in such a way that a plurality of the backup servers are always formed.