PURPOSE: To utilize a boss material effectively so as to crimp at a high speed by inserting a boss protruding from a thermoplastic plastic member into a hole of other member and enlarging the external diameter of the boss, and forming an expanding head on its tip.

CONSTITUTION: A hollow boss 13 of a member 12 is inserted into a through hole 11 of a member 10 so as to butt the members 12 and 10 against each other. Thereafter, an axial core of a crimping chip 1 is aligned with an axial core of the boss 13. When a heated core bar 2 is made to descend and pushed into the hollow of the hollow boss 13, the tip of the hollow boss is heated and softened, the tip of the hollow boss 13 is enlarged by a conical face of a protrusion 4 in accordance with the descent of the core bar 2, and the central part of the external diameter of the hollow boss 13 is butted against the end edge of the through hole 11 during a period that the core bar 2 descends till a predetermined position. Successively, an outer cylinder 3 is made to descend and a bottom face of a groove 5 formed on the outer cylinder 3 is butted and pressed against the tip of the hollow boss 13 which is overhanging from the outer peripheral face of the core bar 2, so that an expanding part is molded in a cavity to be formed by closures of the groove 5 and surface of the member 10.