A heat exchanger which has i) a fin material comprising a triple-layer clad material constituted of a core material composed of an aluminum alloy containing from 0.5 to 1.8% by weight of Mn and from 0.5 to 3.0% by weight of Zn and, provided on both sides of the core material, a brazing filler material composed of an Al--Si alloy containing from 6.5 to 13.0% by weight of Si and from 0.15 to 0.60% by weight of Cu and ii) an aluminum alloy tube having a Zn concentrated surface; the both having been brazed with each other; wherein, after brazing, recrystallized grains of the core material have an average length of from 100 to 1,000 .mu.m in the lengthwise section of a fin and the recrystallized grains of the core material are 4 or less in average number in the thickness direction of that lengthwise section. This heat exchanger is improved in the durability of fin joints and fins themselves and their strength after corrosion.