To provide a plate fin type heat exchanger which improves heat exchange efficiency in the air-cooled plate fin type heat exchanger and, thereby, reduces the weight of the heat exchanger.

A corrugated fin 4 is arranged in a second flow channel 12 in which cooling air flows in a core 1 of a plate fin type heat exchanger and the corrugated fin 4 is arranged so as to be protruded from an inflow edge surface 13 through an inflow opening 14 of the core. The protrusion part constitutes a turbulent flow transition part 42 for changing cooling air that flows in the second flow channel into a turbulent flow.