In a heat exchanger having a core portion in which aluminum flat tubes and aluminum fins are disposed alternatively and brazed with each other, each fin has one side portion located at one side of the tube and the other side portion located at the other side of the tube. The one side portion of the fin is higher in pitting potential than the other side portion of the fin, and the pitting potential difference between the one side portion of the fin and the other side portion of the fin is 40 to 200 mV. The pitting potential difference can be formed by the Zn concentration difference in the aluminum alloy constituting the fin.