A game apparatus stores, when an output value is equal to a boundary value of a predetermined range, boundary angle data representing a boundary angle corresponding to a tilt angle of an input device. The game apparatus obtains, from an input device, data including a value changed in accordance with a tilt angle of the input device so as to calculate the tilt angle of the input device. Thereafter, when a tilt angle calculated in a tilt calculation step is greater than the boundary angle represented by the boundary angle data, the boundary angle data is updated such that the tilt angle calculated in the tilt calculation step becomes a new boundary angle. The output value is calculated based on a ratio of the tilt angle calculated in the tilt calculation step to the boundary angle represented by the boundary angle data.