To completely deal with an individual desire concerning the size of a constituent component of a heat exchanger, particularly a desire concerning height thereof, and reduce even the whole size of the heat exchanger required each time by refining thermal conductance to surrounding air or indoor air from the constituent component.

As a constituent component 1 of a heat exchanger there is provided a part of a polygonal tube extruded from aluminum or an aluminum alloy. The polygonal tube has two side portions 2 opposing to each other and being parallel each other. The constituent components 1 are provided side by side so as to overlap on each other with the aid of the side portions 2 and are combined with each other with the aid of the same. The side portion 2 includes a hollow section forming a passage in an upper region and a lower region. An upper end surface and a lower end surface of the constituent component 1 are closed with a bottom 12 and a cover 13 and are combined with each other.