In a game system for use in a card game by using a plurality of cards which individually have predetermined ability points, a plurality of competition environments are prepared to change the ability points from one to another at every one of the competition environments. Each of the competition environments is determined by a field card. To this end, the game system has a storage device which stores the ability points varied in dependency upon the competition environments. The ability points of each card is stored as attack and defense ability points different from each other and varied by the competition environments. Any other cards, such as magic cards, trap cards, may be prepared.