A heat exchanger comprises a casing designed as a hollow box by a top plate, a bottom plate, a front plate, a rear plate, and two lateral plates and a heat transfer unit accommodated in the casing. The heat transfer unit has flat plates alternately folded back in reverse direction. A first flow passage and a second flow passage are alternately formed in multiple layers between the flat plates. A high temperature fluid inlet and a high temperature fluid outlet in communication with the second flow passage are provided on the front plate side. A low temperature fluid inlet and a low temperature fluid outlet in communication with the first flow passage are provided on the rear plate side. Biasing means applies pressure on the lateral plate sealing member to the end portion of the heat transfer unit and seals the end portions of the first and second flow passages.