The data acquisition means acquires the acceleration data at predetermined time intervals. The change amount calculation means calculates a change amount vector representing a change amount of the acceleration by using the acceleration data having been acquired by the data acquisition means. The accumulation vector calculation means calculates an accumulation vector by sequentially and cumulatively adding the change amount vector having been calculated by the change amount calculation means. The direction determination means determines, as a waving direction representing a moving direction of the input device, a direction of the accumulation vector, when the accumulation vector having been calculated by the accumulation vector calculation means satisfies a predetermined condition.