A heat exchanger for an aircraft engine configured to perform heat exchange with an airflow in the aircraft engine, the heat exchanger (100; 200) comprising: a core configured to allow a fluid to be cooled to flow therethrough; and a plurality of plate-shaped radiating fins (2a, 2b, 2c, 2d; 102; 102a, 102b, 102c, 102d) on a surface of the core; wherein each of the radiating fins (2a, 2b, 2c, 2d; 102; 102a, 102b, 102c, 102d) includes: a first portion (21; 121) configured to rise from the surface of the core; a second portion (22; 122) arranged on a tip side relative to the first portion (21; 121), the second portion (22; 122) having a thickness smaller than that of the first portion (21; 121); and a connecting portion (23; 123) configured to connect between the first portion (21; 121) and the second portion (22; 122), the connecting portion (23; 123) being inclined with respect to the first portion (21; 121) and the second portion (22; 122) and included a pair of side portions on opposite sides in the thickness direction; and at least one of the pair of side portions is an inclined side portion (23a; 23b; 123b) inclined at a different angle from side portions of the first portion (21; 121) and the second portion (22; 122) each connecting to the inclined side portion (23a; 23b; 123b), and the radiating fins (2a, 2b, 2c, 2d; 102; 102a, 102b, 102c, 102d) are configured such that a ratio y/Y of a height y from the surface of the core to a center position of the connecting portion (23; 123) to a total height Y of each of the radiating fins (2a, 2b, 2c, 2d; 102; 102a, 102b, 102c, 102d) from the surface of the core satisfies a relationship of 0.34 < y/Y < 0.64. a core configured to allow a fluid to be cooled to flow therethrough; and a plurality of plate-shaped radiating fins (2a, 2b, 2c, 2d; 102; 102a, 102b, 102c, 102d) on a surface of the core; wherein each of the radiating fins (2a, 2b, 2c, 2d; 102; 102a, 102b, 102c, 102d) includes: a first portion (21; 121) configured to rise from the surface of the core; a second portion (22; 122) arranged on a tip side relative to the first portion (21; 121), the second portion (22; 122) having a thickness smaller than that of the first portion (21; 121); and a connecting portion (23; 123) configured to connect between the first portion (21; 121) and the second portion (22; 122), the connecting portion (23; 123) being inclined with respect to the first portion (21; 121) and the second portion (22; 122) and included a pair of side portions on opposite sides in the thickness direction; and a first portion (21; 121) configured to rise from the surface of the core; a second portion (22; 122) arranged on a tip side relative to the first portion (21; 121), the second portion (22; 122) having a thickness smaller than that of the first portion (21; 121); and a connecting portion (23; 123) configured to connect between the first portion (21; 121) and the second portion (22; 122), the connecting portion (23; 123) being inclined with respect to the first portion (21; 121) and the second portion (22; 122) and included a pair of side portions on opposite sides in the thickness direction; and at least one of the pair of side portions is an inclined side portion (23a; 23b; 123b) inclined at a different angle from side portions of the first portion (21; 121) and the second portion (22; 122) each connecting to the inclined side portion (23a; 23b; 123b), and the radiating fins (2a, 2b, 2c, 2d; 102; 102a, 102b, 102c, 102d) are configured such that a ratio y/Y of a height y from the surface of the core to a center position of the connecting portion (23; 123) to a total height Y of each of the radiating fins (2a, 2b, 2c, 2d; 102; 102a, 102b, 102c, 102d) from the surface of the core satisfies a relationship of 0.34 < y/Y < 0.64.