A sphere having a plurality of marks given to the surface thereof is photographed at a plurality of times to obtain a plurality of two-dimensional images of said sphere. An imaginary sphere having a plurality of marks given to the surface thereof is generated in a three-dimensional coordinate space by a computer. In making the imaginary sphere placed in a reference posture coincident with the posture of the two-dimensional image, an optimum displacement amount of said posture of said imaginary sphere is determined by using a computation based on Genetic Algorithms. A rotation matrix is derived from the posture of the sphere measured for each dimensional image thereof and a computation is performed on the rotation matrix to thereby obtain the rotational amount of the sphere and the direction of its rotational axis.