A heat exchanger includes a vertical pressure vessel having a air-tight structure, one and another sets of fluid inlet/outlet ports provided to upper and lower portion of the vertical pressure vessel, respectively, and a heat exchanging module disposed in the pressure vessel to a portion between both sets of the fluid inlet/outlet ports, the heat exchanging module being formed by stacking a number of heat exchanging elements. At least one set of the fluid inlet/outlet ports are configured to provide a header composed of a pipe structural member having a bottom portion closed and a heat insulating material attached to an inner surface of the pipe structural member, and end portions of fluid flowing pipes communicating with the heat exchanging module are opened at the closed fluid inlet/outlet ports in the header.