A network game system includes a center server and game machine. The center server includes a first transmitter, which transmits, to the game machine, first data indicating a settled play amount out of a played amount which is represented by either an accumulated number or time period that a game program has been executed. The game machine includes a first storage, operable to store the first data; a second storage, operable to store second data indicating the played amount; a first receiver; an updater, which updates the first data stored in the first storage with first data received by the first receiver; a third storage, operable to store third data indicating an allowable unsettled play amount which represents either an allowable number or time period of which the game program is executed without settlement; and a controller, which controls the game machine based on the first, second, and third data.