A heat exchanger utilizes a multi-hole flexible band of light metal which is formed, by extrusion, with a plurality of longitudinal small holes extending in parallel to one another from one band end to the other end. The longitudinal holes are connected at each of the end portions of the band, and both ends of the band is closed by welding to form a sealed cavity partly filled with a working fluid in partial vacuum. The sealed cavity may be in the form of a single long continuous passage, or in the form of parallel passages connected together at both ends. The multi-hole band is bent in such a shape that the band meanders between a high temperature region and a low temperature region. The thus-constructed heat exchanger is advantageous in heat exchanging performance, and capable of reducing the manufacturing and material costs, the weight of the heat exchanger, and improving the reliability.