PURPOSE: To enable free choice of placing works in a furnace and improve work efficiency by providing a clearance for capillary phenomenon and making spot welding at several places.

CONSTITUTION: Spot welding 3a is provided at several places on a heat exchanger 1a without grooving to keep a clearance 1b over the whole section. Powdery flux 4 necessary to fill the whole section in quantity is placed at any place in the clearance 1b. Then, brazing welding is completed over the whole section by capillary phenomenon. Since spot welding 3a is provided on the heat exchanger 1a and sheath heater 2, brazing welding can be made in the furnace 5a piling the works in an optional way such as horizontal piling, slanting piling, vertical piling, reverse piling etc. Capillary phenomenon is caused by melting of the flux 4 and the whole section of the clearance 1b is filled.