PURPOSE: To protect an elastic body while preventing a core from being damaged due to the heat insulating effect of a plastic layer by a method wherein a plastic layer with further excellent heat resistance is laid between the elastic body and the core.

CONSTITUTION: A plastic layer 20 with excellent heat resistance is formed along the periphery of a core 1 and an elastic body 22 is laid between the core 1 and a ring gear 3 through the intermediary of the plastic layer 20, therefore even if the ambient temperature along the periphery of the core 1 rises up to nearly 400°C, the heat value conducted to the elastic body 22 is reduced by the heat insulating effect of the plastic layer 20 restraining the temperature rise of the elastic body 20 restraining the temperature rise of the elastic body 22 and extending the service life of said elastic body 22. On the other hand, the ventilating holes 23 pierced on the border of the elastic body 22 and the plastic layer 20 effectively accelerate the evaporation of a rubber hardener in the forming process providing said elastic body 22 and plastic layer 20 around said holes 23 with cooling effect due to the air on the low temperature atmospheric side passing through said ventilating holes 23.