1. A network game system having a server apparatus and a plurality of client apparatuses each connected to said server apparatus via a network, said server apparatus comprising: a server message receiver that receives messages from each of said plurality of client apparatuses; a server message transmitter that transmits messages on progress of a game including the messages received by said server message receiver to each of said plurality of client apparatuses via the network; and a specific information transmitter that transmits specific information when said server message transmitter transmits a specific message, the specific information indicating that the transmitted message is the specific message, each of said plurality of client apparatuses comprising: a message input device that inputs a desired message on the progress of the game according to a user's operation; a client message transmitter that transmits the messages input from said message input device via said network to cause said server message receiver to receive the messages; a client message receiver that receives messages transmitted from said server message transmitter; a specific information receiver that receives the specific information transmitted from said specific information transmitter; a message display device that displays messages received by said client message receiver while scrolling the messages sequentially; and a display controller that controls scroll-display displayed by said message display device such that the display of the message corresponding to specific information received by said specific information receiver is prevented from being erased from a display area on the display device until a predetermined condition is satisfied.