PURPOSE: To enable to perform a welding-in-place process corresponding to the shape of a connection surface and to weld resin at an inexpensive cost, by a method wherein a thin electric resistor is inserted between hot-melt resin A and hot-melt resin B, the resistor is heated by energizing it to melt the two resins, and they are cooled for curing.

CONSTITUTION: An electric resistor 10, consisting of a thin metal sheet and fine resistance wire, is inserted between connecting surfaces C of hot-melt resin A and hot-melt resin B which consist of, for example, a thermoplastic high molecular material, such as polyethylene, polypropylene, and energizing parts 11 and 12 are charged with a current to heat the resistor to melt the two resins at the connecting surface. The resistor stops being energized, and the connecting surface is cooled and cured for connection. In addition, it is advisable to use said electric resistance wire which is formed in a vortex, or to use a porous electric resistance sheet which is formed in the shape matching that of a connecting surface.