A network game system in which a server-cum-device is changed while not marring the excitement of a game. A network game system (10) allows game machines (12) to act as clients and at least one of the game machines (12) to act as a server. At a change judgment timing, a change judging section (52) judges on the basis of the play evaluation value corresponding to at least one of the game machines (12) whether or not the change judging section (52) changes the game machine (12) acting as a server to another one. If the change judging section (52) judges that it does not changes the game machine (12) acting as a server, a change judgment inhibiting section (56) inhibits formation of judgment by the change judging section (52) until a predetermined change inhibition period elapses.