A heat exchanger is constructed by laminating rectangular shaped partition walls and corrugated fins alternately. A first fluid passage through which a high temperature internal air and a second fluid passage through which the low temperature external air flows are formed into rectangular parallelopipe-shape the longitudinal direction of which is upper and lower direction. The corrugated fin is disposed inside the fluid passage to incline frontwardly. A first fluid inlet is provided at an upper position and a first fluid outlet is provided at a lower position of the first fluid passage. While, a second fluid inlet is provided at a lower position and a second fluid outlet is provided at an upper position of the second fluid passage. That is, the first fluid and the second fluid flow in an opposite direction to each other. Therefore, because the fluid introduction area of the corrugated fin is large in comparison with that in a conventional counter-flow type heat exchanger, the pressure loss of the fluid is suppressed, and the corrugated fin does not tend to be choked with a dust or the like.