A game apparatus calculates a gravitational vector Vg, which represents a gravitational direction viewed from an input device, based on operation data. Then, a motion acceleration vector VA, which represents an acceleration applied by a motion of the input device, is calculated based on an acceleration represented by acceleration data and the gravitational vector Vg. The gravitational vector Vg is corrected such that the motion acceleration vector VA approaches a motion acceleration (vector VA′) satisfying a relationship with an angular rate (vector Vω) represented by angular rate data, the relationship being predefined between the motion acceleration and the angular rate for the input device making a predetermined motion (rotational motion). Furthermore, the game apparatus calculates an orientation of the input device corresponding to the corrected gravitational vector.