PURPOSE: To surely allow brazing material to flow into gap and to prevent insufficient sticking by arranging notch part having dimension a little larger than diameter of the brazing filler rod parallel to side plates at both shoulder parts of fins joining with the side plates and forming inserting grooves for the brazing filler rod.

CONSTITUTION: Combustion gas generated with a burner 9 is passed through a combustion chamber 8 and gaps between fins in a heat exchanger 7 and discharged. Water in a pipe receives heat through the fins and the heat exchange pipe 6 and is discharged as high temp. water. When sticking of the fins to the side plates is insufficient, hole is developed by high temp. combustion gas. For this countermeasure, the positions putting the brazing filler rod for sticking the fin to the side plate are formed by arranging the notching parts parallel to the side plates, therefore even if the brazing filler rod bends, the brazing filler rod is brought into contact with the side plate. By this method, at the time of welding with the brazing filler rod, the brazing material is surely allowed to flow into the gap between the fin and the side plate and insufficient sticking of the gap between the fin and the side plate is not developed.