原著論文挿図Spinning motions of a gyroscope are analyzed based on its 3D moving images to measure the inclination angle, the precession angular velocity and the spin angular velocity in various stages of its motion. As a result of averaging over a short time period, it is found that the popular theoretical formula for the precession angular velocity, which means the constancy of product of the precession angular velocity and the spin one, holds when the ratio of the spin angular velocity to the precession one is larger than 25 at most, and that it breaks down with the product constant decreased when the ratio is smaller than 20 at least.