A game controller includes at least four load sensors for detecting a load applied on a support board on which player's legs are ridden, and the game controller transmits a load value detected as manipulation data by the four load sensors to a game machine. The game machine determines a necessary quantity of load values, and the game machine computes the necessary quantity of load values based on the detected load value from the game controller. Then, game processing is performed based on the necessary quantity of computed load values.