A tennis game system includes a game machine connected to a television set via an AV cable, and a racket-shaped input device for inputting operation to the game machine. A game player instructs a ballstriking player to strike a ball on a monitor screen by operating the racket-shaped input device. At this time, a game processor included in the game machine calculates a predicted return position ofa ball returned by the opposite player, compares a current position of the ball striking player with the predicted return position, and judges whether the predicted return position is within a ball strikable range for the ball striking player. If a judgment means judges that the predicted return position is out of the ball strikable range, a ball striking position movement means, i.e. the game processor moves the ball striking position.