Each time a player character performs a boom triggering act, "1" is added to the corresponding act number counter provided for each non-player character. The value of each act number counter is periodically added to a boom value provided for each non-player character. When a boom value reaches an upper limit, the corresponding non-player character performs an act related to the boom triggering act performed by the player character. Thus, the act of each non-player character is changed in accordance with the act performed by the player character in the game world, which provides the player with a fresh feeling that the act performed by the player character set a trend in the game world.