A game system which enables a host player to progress a game in collaboration with guest players is provided. The host player progresses the game by moving a party, including a plurality of player characters, on a field. When the party arrives at a predetermined point on a predetermined field, a battle with an enemy character is started. When a player character of the party becomes inoperative in the battle, a player character of a guest player may be substituted for the inoperative player character by sending a support request to the guest player. Although there is a time limit that the player character of the guest player may participate in the battle, the time limit may be extended when the corresponding player character achieves a predetermined condition in the battle.