PURPOSE: To easily hold the inner ball floatingly with high precision by controlling apparent specific gravity of a part, by utilizing the magnetic fluid as a holding liquid.

CONSTITUTION: Magnets M1, M2,...Mn are adhered on the external wall of a outer ball 1 with adhesive. These many magnets M1, M2,...Mn are located so that mutual magnetic fluxes are placed in a form of a ring magnetically. When a distance between magnets is separated, magnetic field is extended to the inner ball 3 side, and when the distance is decreased, magnetic field becomes thin and influence to the magnetic fluid 2 can be decreased. As many magnets are used, total density of magnetic field can be enlarged and floating condition of the inner ball 3 can always be kept without having adverse influence caused by rising of temperature.