PURPOSE: To provide a receiving mouth mold which facilitates a receiving-mouth having a stabilized receiving mouth in a desired dimension, even when adapted to the molding of the receiving mouth of a thermoplastic resin pipe P using a crystalline resin such as polyethylene as a material.

CONSTITUTION: An inner mold 1 provided with a small diameter part 11 of which the outer diameter is almost equal to the inner diameter of a thermoplastic resin pipe P, a large diameter part 13 of which the outer diameter is almost equal to the inner diameter of a receiving mouth to be molded and the taper part 12 allowing the small and large diameter parts 11, 13 to continue smoothly is provided. Further, an outer mold 2 formed form upper and lower molds 2a, 2b and having a third molding surface 24 of which the inner diameter is almost equal to the outer diameter of the receiving mouth to be molded provided thereto at the position opposed to the large diameter part 13 and an intermediate mold 3 having a cylindrical part 31 of which the outer diameter is larger than the inner diameter of the receiving mouth to be molded and slidable in the axial direction of the large diameter part 13 along the outer surface of the large diameter part 13 of the inner mold 1 are provided.