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 message collector that receives messages input in each of said plurality of client apparatuses and transmitted therefrom; and a message distributor that distributes messages in accordance with progress of a game including the messages received by said message collector to each of said plurality of client apparatuses via the network; and each of said plurality of client apparatuses comprising: a message input device that inputs desired messages in accordance with the progress of the game according to a user's operation; a message transmitter that transmits the messages input from said message input device to said server apparatus via said network to cause said message collector to receive the messages; a message receiver that receives messages transmitted from said message distributor; a message display device that scrolls and displays messages received by said message receiver in a window displayed on a game screen; a window enlarging device that enlarges the window in response to reception of the message by said message receiver so as to increase the number for display lines of the messages in the window up to a preset maximum value; and a window reducing device that reduces the window so as to decrease the number of display lines for the messages in the window to a preset minimum value when a predetermined condition is satisfied.