To realize the stable soldering excellent in flow-out of the flux and wettability of the molten solder by irradiating the light beam to the tip of a thread solder in which the flux is contained and a stitch is present on an outer circumferential surface, and melting the thread solder for soldering.

A thread solder 30 contains the flux 31 in the center part, and forms a stitch 32 in an outer circumferential surface. The tip of the thread solder 30 brings a work 27 close to a part to be soldered of a work 28 by pulling a holed member 48 by the force of a spring 49. When the part to be soldered is irradiated with the light beam 10, the flux 31 flows out excellently from the thread solder 30 through formation of the stitch 32, and the molten solder follows the flux and expanded over the works 27, 28. By setting the direction of irradiation of the light beam 10 to the direction in which the shadow of the thread solder 30 is formed on the work 28 of smaller heat capacity, the heat balance with the work 27 is realized to provide the stable soldering.