To prevent the wall thickness of a groove part becoming less than that of the other part of a receiving mouth when a receiving mouth having a groove provided to the inner peripheral surface thereof is formed to the end part of a thermoplastic resin pipe.

A receiving mouth processing mold is equipped with an inner mold 10 having the insertion guide part 11 capable of being inserted in the end part of a thermoplastic resin pipe at its leading end and formed into an outer shape almost same to the inner surface shape of a receiving port and an outer mold constituted of two split molds 20a, 20b and formed into an inner surface shape almost same to the outer surface shape of the receiving port by clamping the split molds 20a, 20b. The inner mold 10 is formed from a movable mold 12 capable of expanding and contracting the diameter of a groove diameter expanding part at least forming the groove of the receiving mouth and the press mold 30 compressing the end part of the thermoplastic resin pipe inserted in the inner mold 10 in the pipe axial direction is provided along the outer surface of the inner mold 10 in a slidable manner.