Abstract: The heat exchanger uses multi-layer tubes, performing efficient heat exchange. Each heat transfer tube includes an outer tube and an inner tube. Header unit directs first fluid through gaps between the inner tube and the outer tube from outer circumferences of the inner tube. The header units are stacked in a top-bottom direction with gap-forming members therebetween, leaving predetermined gap portion. Furthermore, a header cover directs the second fluid through the inner tubes extending from the header units, and is fitted from the outer side, and the second fluid is introduced. With this configuration, the second fluid flowing from the header cover is directed through the inside of the inner tubes and over outer surfaces of the outer tubes along the axial direction via the gap portions, enabling heat exchange to be performed from both inside and outside of the outer tubes.