To flexibly cope with various network configuration and to allow each process to continue a desired processing even on the occurrence of a fault on a network.

The network system is a system where processes P1-P5 that can mutually transmit/receive a message by a broadcast not designating a destination and a message designating a specific process section and can change their own states into those of master and slave processes are interconnected. For example, in the case that the process P1 acts like a master process and the processes P2-P4 act like slave processes and a group consists of a processes P1-P4, each of the slave processes P2-P4 stores the master process P1 and the master process P1 stores each of the slave processes P2-P4 that stores the master process P1. In the case that a process P5 of other group is connected to this group, the master process P1 and the process P5 exchange a message for negotiation and either of them acts like the master process and the other is changed into the slave process.