PURPOSE: To easily position a metallic ball at the center of a synthetic resin ball thereby to reduce manufacturing costs remarkably, by heating the synthetic resin ball with high frequencies in the state where the centrifugal force is impressed to the ball, and cooling the ball afterwards.

CONSTITUTION: A predetermined number of track balls 11 involving metallic balls 22 are supplied onto a bottom part 10a of a rotary disc 10 by injection molding. When a motor 12 is driven to rotate tame rotary disc 10, the track balls 11 at the bottom part 10a are moved to a peripheral wall 10b by the centrifugal force. The track balls 11 roll over while rotating along the peripheral wall 10b, so that coil driving devices 16a, 16b are concurrently driven. As a result, a heating coil device 14 is descended and stopped in a predetermined position. At the same time, a high frequency generating device 21 is activated to feed a predetermined high frequency current to the heating coil device 14.