A game system comprising a game apparatus, a first game controller comprising a first gyro sensor, a second game controller comprising a second gyro sensor, an accessory to and from which the game apparatus, the first game controller and the second game controller are attachable and detachable, and game processing means comprised in the game apparatus for executing game processing, the accessory comprising: a first fixing portion configured to rotatably fix the first game controller to an accessory main body so as to rotate about at least a certain axis; and a second fixing portion configured to fix the second game controller to the accessory main body; and a third fixing portion configured to fix the game apparatus to the accessory main body, the game system further comprising: first calculation means (81, S164) for, based on an output from the first gyro sensor of the first game controller fixed to the first fixing portion, calculating a first value indicating an orientation of the first game controller; second calculation means (81, S164) for, based on an output from the second gyro sensor of the second game controller fixed to the second fixing portion, calculating a second value indicating an orientation of the second game controller; and third calculation means (81, S165) for calculating a difference value indicating a difference between the first value and the second value, the game processing means comprising first game processing means (81, S166) for executing the game processing executes a first game process based on the difference value. the accessory comprising: a first fixing portion configured to rotatably fix the first game controller to an accessory main body so as to rotate about at least a certain axis; and a second fixing portion configured to fix the second game controller to the accessory main body; and a third fixing portion configured to fix the game apparatus to the accessory main body, a first fixing portion configured to rotatably fix the first game controller to an accessory main body so as to rotate about at least a certain axis; and a second fixing portion configured to fix the second game controller to the accessory main body; and a third fixing portion configured to fix the game apparatus to the accessory main body, the game system further comprising: first calculation means (81, S164) for, based on an output from the first gyro sensor of the first game controller fixed to the first fixing portion, calculating a first value indicating an orientation of the first game controller; second calculation means (81, S164) for, based on an output from the second gyro sensor of the second game controller fixed to the second fixing portion, calculating a second value indicating an orientation of the second game controller; and third calculation means (81, S165) for calculating a difference value indicating a difference between the first value and the second value, first calculation means (81, S164) for, based on an output from the first gyro sensor of the first game controller fixed to the first fixing portion, calculating a first value indicating an orientation of the first game controller; second calculation means (81, S164) for, based on an output from the second gyro sensor of the second game controller fixed to the second fixing portion, calculating a second value indicating an orientation of the second game controller; and third calculation means (81, S165) for calculating a difference value indicating a difference between the first value and the second value, the game processing means comprising first game processing means (81, S166) for executing the game processing executes a first game process based on the difference value.