PURPOSE: To eliminate the need for a device to provide pressure difference between the interior and exterior of original tube and consequently reduce the manufacturing cost of the title heat-shrinkable tube by a method wherein a metal piece, the diameter of which is larger than the inner diameter of original tube, is inserted in the heated original tube, so as to be shifted relatively downwards, by feeding the original tubing vertically or from below to above.

CONSTITUTION: Firstly, original tube 1 is heated in advance by a heater 3 up to a temperature exceeding the softening temperature of the tube. Secondly, a metal sphere 2 is inserted in the original tube 1 and at the same time the original tube 1 is fed vertically from the lower part of a guide 4 along a small diametral part 411 so as to be shifted upwards while being heated in the guide 4 continuously connected to the inside of the heater 3 in order to obtain heat-shrinkable tube 1', which is expanded and enlarged radially. In addition, as the original tube 1 shifts upwards while being expanded along the tapered part 413 of the guide 4, the metal sphere 2 inserted in the original tube 1 tends to shift upwards because of the frictional resistance developed between the film surface of the heat-shrinkable tube 1' and the metal sphere 2. However, the own weight of the metal sphere causes the metal sphere to slide in the heat- shrinkable tube 1' down to the tapered part 413 so as to be held by the tapered part 413.