PURPOSE: To finish the inner peripheral surface of a small diameter fixed part into complete sliding surface so as to improve sealing performance to the shaft of a joint such as a constant velocity joint.

CONSTITUTION: Fused resin is injected into an injection metal mold cavity 3, formed by a small diameter fixed part metal mold 1 and a core metal mold 2, from a fused resin path 6 formed by a die 4 and a mandrel 5. The small diameter fixed part metal mold 1 and the core metal mold 2 are then moved upward between opened blow metal molds 7, 7, and a parison 12 is formed and disposed between the blow metal molds 7, 7. The blow metal molds 7, 7 are then closed for blow molding so as to form a small diameter fixed part provided at its outer periphery with a recessed part for mounting a clamping means, by the injection metal mold cavity 3.