A heat exchanging system (10) comprises a heat exchanger (1) including a tube (2) through which a first fluid flows and a plurality of fins (3) made from thin plates attached to the tube (2) and arranged parallel to each other in the direction along which the tube (2) extends, and a fan (4) for introducing a second fluid between the fins (3). The fin (3) includes concave parts (7) and convex parts (8) continuously and cyclically formed in a zigzag line. The concave parts (7) and the convex parts (8) are so arranged as to extend in the direction crossing the flow direction of the second fluid flowing between the fins (3), and the flow of the second fluid flowing between the fins (3) is made to be cyclically variable.