Ends of heat-transfer plates S1, S2, formed by bending folding plate blanks in a zigzag fashion along folding lines L1, L2, are cut in an angle shape, and flange portions 26 formed by folding apexes of the angle shape are superposed one on another and brazed in a surface contact state, thereby to form combustion gas passage inlets 11 and air passage outlets 16 along the two end edges of the angle shapes. Compared with brazing of separate flange members onto the cut surfaces of the apexes of the angle shapes, this fabrication not only dispenses with precise finishing of the cut surfaces, but also serves to increase the brazing strength.