To lower the manufacturing cost of a seal ring by forming a sealing member to be set between a projected strip and a recessed groove from a hollow rubber material having excellent elasticity.

Setscrews are screwed in the front circumferential rim of a closed cover body 21 toward the front aperture c of a rear part casing 15b at prescribed intervals, a hollow seal ring 30a is pressurized by a projected strip 27 in a recessed groove 26, and further the closed cover body 21 is fastened to the rear part casing 15b, so that strain is generated in the fastening part P of the closed cover body 21 through the hollow seal ring 30a in the rear part casing 15b side. As a result, due to the straining phenomenon of the closed cover body 21, the part of the seal ring 30a near the fastening part P is warped (contracted) and to the contrary, the parts besides the fastening part P are expanded. Consequently, the seal ring 30a is firmly attached to the projected strip 27 in excellent state, so that the joining part 42 of the closed cover body 21 and the front aperture part c of the rear part easing 15b can smoothly be sealed.