PURPOSE: To finish the inner surface of a joint part smoothly by taking a core out of a pipe material after disassembling it into three parts or more in the peripheral direction so as to make the clearance small between the outer diameter of the core and the pipe material and thereby preventing molten resin from overflowing into the inside of the pipe material.

CONSTITUTION: After disassembling, a male member 31 is taken out of pipe materials P1, P2. In the case of disassembling the male member 31, the pipe materials P1, P2 are inclined after pulling a coated wire 33 and taking out the core material 31. Namely, when the core material 31 is extracted, one of the four parts 31b, 31c, 31d, 31e surrounding the core material 31 falls in the space defined after extracting the core material 31, and thus when the pipe materials P1, P2 are Inclined, the remaining three parts 31c, 31d, 31e also lose their balance and are disassembled. In this manner, after mutually joint-welding the pipe materials P1, P2, since the male member 31 is taken out of the pipe material P2 after disassembling, molten resin can positively be prevented from overflowing into the inside of the pipe material P1, P2.