A flat tube brazing method for a laminated heat exchanger, having the steps of laminating a plurality of solder-clad flat tubes (2) via fins (3), inserting both end portions of a flat tube or at least one end portion thereof into a tube insert hole (5) of a header tank (4) and combining them, and unitarily brazing the end portion(s) of the flat tubes and tube insert hole together, characterized in that the width A of the tube insert holes is set greater than that B of the flat tubes, the laminated heat exchanger being placed laterally so that the widthwise axes of the flat tubes extend vertically, and supported by a jig (19) so that a clearance (20) is formed in the portion of the tube insert holes which are below the lower portions of the flat tubes, at the time of the unitary brazing of the combined flat tubes. Owing to this method, the inserting of the flat tubes into the holes at the time of combining thereof is done reliably, and the brazing performance of the tube insert holes and flat tubes can be improved. The brazing of beads is also done reliably, so that a laminated heat exchanger in which the pressure resistance is secured is obtained.