To provide a highly reliable heat exchanger for an aircraft suppressing the generation of distortion caused by thermal stress by simple structure and hardly generating crack.

The heat exchanger for the aircraft is provided with a heat conductive core 60 having a high-temperature flow passage 66 making a high-temperature side inlet part 63 communicate with a high-temperature side outlet 64 and a low-temperature flow passage 65 making a low-temperature side inlet 61 communicate with a low-temperature side outlet 62 and performing heat exchange between the low-temperature flow passage 65 and the high-temperature flow passage 66; a high-temperature side duct 33 connecting the high-temperature flow passage 66 of the core 60 with a high-temperature side pipe 72; and a square tube-shaped low-temperature side duct 31 connecting the low-temperature flow passage 65 of the core 60 with low-temperature side pipe 71. One end of the low-temperature side duct 31 is integrally jointed to the core 60, and the other end is connected to the low-temperature side pipe 71 via a square tube-shaped flange 80a. The square tube-shaped flange 80a continued to the low-temperature side duct 31 of the heat exchanger for the aircraft is divided into corners 81 and side walls 82.