PURPOSE: To join reliably the whole surface by applying necessary and sufficient pressure to the whole surface of a joining part, by a method wherein split molds each are expanded or contracted each other independently according to restoration of split supporting parts each corresponding to those to stored forms under a state where an inside and outside materials are superposed upon each other between an inner and outer molds.

CONSTITUTION: Respective setting positions of a large number of split molds 18 constituting an outer mold 13 are displaced through a form change of split supporting parts 17 to be performed with a temperature adjusting part 19. The split molds 18 each contract independently their diameters each other according to restoration of the split supporting parts 17 each corresponding to those to stored forms under a state where lid paper 12 and a paper cup 14 are superposed upon each other between an inner and outer molds 11, 13. In the case where, a joint part 15 forming a projected from is provided on a part of a circumferential direction of the paper cup 14, the split molds 18 each not only those corresponding to the joint part 15 but also those corresponding to the other part can pressurize a joining part between those lid paper 12 and paper cup 14 with almost the same pressure. Since the respective split molds 18 can pressurize the joining part between the lid paper 12 and paper cup 14 independently with each other, joining can be completed by pressurizing reliably also a triangular stepped part 20.