It is an object of the present invention to obtain a compact and high performance heat exchanger low in its pressure loss of fluid and a refrigerating air conditioner. The heat exchanger of the present invention is a heat exchanger (10) including a first flat tube (1) in which a low temperature fluid flows and a second flat tube (2) in which a high temperature fluid flows that is laminated on the first flat tube and arranged so that the flowing direction of the high temperature fluid is parallel to the flowing direction of the low temperature fluid. The first flat tube and the second flat tube are folded back in such a way that the first and second flat tube come into contact with each other on their flat surfaces. A plurality of flat tubes is arranged in the direction of lamination. Parallel passages are formed by the plurality of the flat tubes and an inlet header and an outlet header. Either of the inlet header or the outlet header is formed with a tubular header. The plurality of the flat tubes forming the parallel passages are bundled and connected to the tubular header so that the axial direction of the tubular header is the same as the flowing direction of the fluid in the flat tube.