PURPOSE: To improve the surface roughness of the surface of a sphere and improve its strength by pressing a circular die after machined to said surface for its rotation.

CONSTITUTION: In a space where approximately semi-circular concave grooves 5 of a plurality of circular dies 3 are opposite to each other, there is a workpiece 1 placed, and the spherical surface 2 of a sphere formed on the workpiece 1 in a pre-machining process is contacted by a part of the grooves 5 like covering, so that the circular dies 3 are given a rotational motion and the grooves 5 are pressed to the spherical surface 2 for rotation. By this pressed rotation, the surface roughness of the spherical surface 2 becomes equivalent to zero and the surface layer forming the above-mentioned surface 2 is strengthened.