annular shape and a second housing 43 having an annular shape that is fused to the first housing 42 thereby forming an annular housing corresponding to the recess 15 see fig2 of the spin tub 10 the first and second housings 42 and 43 may be for example formed of synthetic resin such as plastic by injection molding as illustrated in fig9 the first housing 42 has a cross section of an approximately u201c c u201d shape includes fusion ridges 42 a protruding to the second housing 43 at opposite ends thereof which are coupled with the second housing 43 and inner pocket ridges 42 b protruding to the second housing 43 spaced inwardly apart from the fusion ridges 42 a the second housing 43 which is coupled to opposite ends of the first housing 42 in order to form a closed internal space 40 a for holding a plurality of balls 41 and a viscous fluid includes fusion grooves 43 a recessed along edges thereof so as to correspond to the fusion ridges 42 a outer pocket flanges 43 b and guide ridges 43 c the outer pocket flanges protrude to the first housing 42 on outer sides of the fusion grooves 43 a so as to be spaced apart from the fusion ridges.