PURPOSE: To prevent a molding material from remaining as flashes on the surface of the part of an insert where the molding material should not be applied.

CONSTITUTION: Insert fitting space 44 for arranging an insert to molding space 13 is formed by upper and lower molds and escape grooves 45 communicating with the open air are formed to the inner peripheral surfaces forming the insert fitting space 44 of the molds in the vicinity of the molding space 13.