PURPOSE: To prevent the body from being cracked or broken for improving durability by burying a reinforcing cloth inside near an annular groove engaging with an attaching hole in an attaching member of the body made of rubber.

CONSTITUTION: An annular groove 10 roughly in form of letter C in section is formed, with which the edge of a fitting hole in the center member is engaged, nearly in the middle of outer periphery of the body 7 made of rubber, and nylon cloth 21 used as a reinforcing cloth is buried along the inner periphery surface 10a of the annular groove 10 inside near the same. Therefore, the deterioration of rubber due to repeated elastic deformation is prevented, so the durability can be improved through preventing the body from generating crack or breakage.