To provide a heat exchanger with improved strength of plates.

In this heat exchanger with first and second tank portions 26, 28 formed on both end portions inside of a sealed housing 20 by joining a top portion and a trough portion formed on a bent portion 31 to inner peripheral faces of plates 21, 22, a plurality of internal fluid passages 27 are formed to communicate the first and second tank portions 26, 28 with each other, and an inlet portion 24 and an outlet portion 25 are formed on the sealed housing 20. The first fluid flows to the inside of the sealed housing 20 through the inlet portion 24, the first tank portion 26, the internal fluid passage 27, the second tank portion 28 and the outlet portion 25. The second fluid flows through a narrow tube 11, to exchange heat between the first fluid and the second fluid, two sheets of plates 21, 22 are provided with projecting ribs 21c, 22c projecting inward in the top portion direction of a corrugated fin 30. Thus, the strength of the plates can be improved.