A cylindrical case main body, and a connecting ring body fitted and fixed to a first end of the case main body and having an annular groove open in the centrifugal direction, are provided. The connecting ring body has a notch open to a side opposite to a second end of the case main body. Into the notch of the connecting ring body, a reinforcement body is fitted, and the reinforcement body has an arc-shaped groove continuing to the annular groove of the connecting ring body in a state where the reinforcement body is fitted into the notch of the connecting ring body. In addition to attaining high rigidity, reduction in weight and reduction in manufacturing cost can also be achieved.