An inclination calculation apparatus sequentially calculates an inclination of an input device operable in terms of a posture thereof. The input device includes acceleration detection means and imaging means. The inclination calculation apparatus sequentially calculates first inclination information representing an inclination of the input device from positions of two imaging targets in a taken image obtained by the imaging means. The inclination calculation apparatus also sequentially calculates second inclination information representing an inclination of the input device from an acceleration detected by the acceleration detection means. The inclination calculation apparatus calculates an inclination of the input device using the first inclination information and the second inclination information.