Abstract: A plate fin heat exchanger adapted to applications using superhigh-pressure fluids was fabricated by brazing, the heat exchanger using as fin stock an alloy consisting essentially of 0.3 to 1.0 wt. % Si, 0.05 to 0.25 wt. % Cu, 0.6 to 1.5 wt. % Mn and 0.45 to 0.9 wt. % Mg, the balance being aluminum and impurities, wherein said impurities contain up to 0.8 wt. % Fe.At least one component selected from the group consisting of 0.05 to 0.25 wt. % Cr, 0.01 to 0.25 wt. % Ti, 0.03 to 0.25 wt. % Zr and 0.01 to 0.25 wt. % V may be added to the above fin alloy stock.By using the above plate fin, excessive silicon diffusion from a brazing alloy into the fin, which may cause reduction of width of the brazed joint, is suppressed, and as a result, lowering of the bonding strength of the brazed joint is eliminated.