To reduce processing cost while smoothing the inner surface of a receiving mouth when the receiving mouth is formed to the end part of a thermoplastic resin pipe.

A receiving mouth processing mold is constituted of a mold main body 10 formed from a cylindrical elastomer elastically deformable in its diameter direction and having two ridges 11, 12 provided to the outer peripheral surface thereof and a diameter expanding core 20 formed from a shaft member having an outer diameter larger than the inner diameter of the mold main body 10 and having a taper guide part 21 of which the leading end outer diameter is smaller than the inner diameter of the mold main body 10 at the leading end part thereof and a split cylindrical reinforcing plate 13 formed from a steel plate is provided along the inner peripheral surface of the mold main body 10.