A heat exchanger comprising: a first flat tube of a flat shape having a through hole in which a low temperature fluid flows; a second flat tube of a flat shape having a through hole in which a high temperature fluid flows; a first inlet header and a first outlet header respectively connected to both ends of the first flat tube; and a second inlet header and a second outlet header respectively connected to both ends of the second flat tube, wherein the first flat tube and the second flat tube are folded back in such a way that the first flat tube and the second flat tube come into contact with each other on their flat surfaces and a flowing direction of the low temperature fluid is parallel to a flowing direction of the high temperature fluid, and are laminated and arranged with a plurality of laminated layers of three or more, characterised in that: at least one of the first flat tube and the second flat tube includes a plurality of flat tubes arranged along the flat surface, wherein the plurality of the flat tubes, and the inlet header and the outlet header respectively connected to both ends of the plurality of the flat tubes form parallel passages, wherein either of the inlet header or the outlet header includes a tubular header having both ends opened, wherein the plurality of the flat tubes that form the parallel passages are bundled and connected to the opened end of the tubular header so that the axial direction of the tubular header is the same as the flowing direction of the fluid in the plurality of the flat tubes. a first flat tube of a flat shape having a through hole in which a low temperature fluid flows; a second flat tube of a flat shape having a through hole in which a high temperature fluid flows; a first inlet header and a first outlet header respectively connected to both ends of the first flat tube; and a second inlet header and a second outlet header respectively connected to both ends of the second flat tube, wherein the first flat tube and the second flat tube are folded back in such a way that the first flat tube and the second flat tube come into contact with each other on their flat surfaces and a flowing direction of the low temperature fluid is parallel to a flowing direction of the high temperature fluid, and are laminated and arranged with a plurality of laminated layers of three or more, characterised in that: at least one of the first flat tube and the second flat tube includes a plurality of flat tubes arranged along the flat surface, wherein the plurality of the flat tubes, and the inlet header and the outlet header respectively connected to both ends of the plurality of the flat tubes form parallel passages, wherein either of the inlet header or the outlet header includes a tubular header having both ends opened, wherein the plurality of the flat tubes that form the parallel passages are bundled and connected to the opened end of the tubular header so that the axial direction of the tubular header is the same as the flowing direction of the fluid in the plurality of the flat tubes. at least one of the first flat tube and the second flat tube includes a plurality of flat tubes arranged along the flat surface, wherein the plurality of the flat tubes, and the inlet header and the outlet header respectively connected to both ends of the plurality of the flat tubes form parallel passages, wherein either of the inlet header or the outlet header includes a tubular header having both ends opened, wherein the plurality of the flat tubes that form the parallel passages are bundled and connected to the opened end of the tubular header so that the axial direction of the tubular header is the same as the flowing direction of the fluid in the plurality of the flat tubes.