A first plate and a second plate have brazing paste on the surface thereof. When the first plate is combined with the second plate while an inner fin is disposed therein, a protrusion forms a clearance there between. Heat is added to evaporate the organic substances in the brazing paste. The organic substances vapor is discharged into outside of the tube through the clearance. After that, heat is added furthermore to melt the brazing paste. The first plate and the second plate are brazed together to form the tube and the inner fin is brazed to the inside of the tube.