Abstract: When aluminium products are preheated in a preheating furnace up to about 500° C. before they are brazed with a flux in a heating furnace, a nitrogen atmosphere in the preheating furnace is made convectional by fans for improving heating efficiencies and also for having the oxygen which is brought into the preheating furnace as a disturbance thereof and scattered about in the nitrogen atmosphere, contacted to furnace inner walls of the preheating furnace which are made from carbonaceous materials, and reacted therewith so as to be changed to CO, resulting in keeping the oxygen concentration of nitrogen atmosphere low, whereby when the aluminium products are brazed by heating them in a continuous atmosphere furnace including said preheating furnace and said heating furnace, heating time thereof is shorten and the brazing is made more economically than before.