Abstract: A plate fin heat exchanger having a first flow channel and a second flow channel which are formed by at least three flat plates arranged in parallel with one another at a predetermined spacing and opposed side walls provided between the adjacent flat plates. A platelike wall interconnecting the side walls is provided within at least one of the first and second flow channels and has a multiplicity of projections arched when seen from one side and formed by cutting the interconnecting wall and being projected upward and downward. Each of the arched projections is opposed to a fluid passage.