To efficiently secure the outer diameter accuracy of a narrow diameter section of a convergent multi-lumen tube and the inner diameter accuracy of a plurality of holes.
A molding method for a thermoplastic tube, in which an end of the thermoplastic tube 11 with holes is drawn finely, is constituted of a process of inserting core bars 12A, 12B and 12C having the outer diameter smaller than the diameter of holes and longer than the length of an end 2 of the thermoplastic tube 11 after being molded into the holes of the thermoplastic tube 11, a process of heating a cylindrical die 13 with an introduction opening 13a of the conical shape to the given temperature and a process of passing the end of the thermoplastic tube 11 into a smallest inner diameter section 13b of the heated molding die 13 to draw the outer diameter of the end finely to the inner diameter dimension of the smallest inner diameter section 13b.
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