Firstly, using a cutting tool 30, connecting ribs 13 are cut off from the end face of a duplex pipe 10 which is held with an outer pipe 11 chucked (rib cutting step). Next, the duplex pipe 10 is cut at and from the end face of the duplex pipe 10 up to the portion thereof corresponding to the exposed length of the inner pipe 12 to thereby form a slit 21 in the outer pipe 11 and connecting ribs 13 (slit forming step) The cutting tool 30 comprises a guide hole 31 into which the inner pipe 12 can be inserted while guiding the inner pipe outer peripheral surface 12a, a cutting device 32 for cutting the connecting ribs 13, and a guide pin 33 which is disposed in the guide hole 31 and can be inserted into the inner pipe 12 while guiding the inner pipe inner peripheral surface 12b.