A heat exchanger including a first header tank and a second header tank that are disposed to be spaced apart a predetermined distance in a height direction and a core part that is disposed between the first header tank and the second header tank and includes a plurality of tubes and fins, the first header tank including a first header plate, a first tank, and a first partition wall that divides a space formed by a combination of the first header plate and the first tank to form a plurality of flow paths, a manifold including an inflow passage and an outflow passage being connected to an outer side of the first header tank, and the inflow passage and the outflow passage having different sizes to each other, the outflow passage having a larger cross-sectional area than that of the inflow passage.