An inner fin (1) of a heat exchanger has, on front and rear faces thereof, protruding ridges (2, 3) protruding in opposite directions to each other. Grooves (4, 5) to serve as passages of a heat exchange medium are formed between the adjacent protruding ridges (2, 3). In wall portions (6) forming these protruding ridges (2, 3), cutout windows (10, 11) are formed to allow the adjacent passages to communicate with each other. Further, protruding weir portions (12, 13) are provided at bottoms of entrances of the cutout windows (10 and 11), so that the heat exchange medium hits against the weir portions to promote diffluence and stirring of the heat exchange medium, thereby enhancing heat exchange efficiency.