A fluid passing flat hollow body 2 comprises an upper and a lower flat plate 15, 16 elongated laterally and spaced apart as positioned one above the other, and a channel forming body 17 interposed between and brazed to the two flat plates 15, 16. The channel forming body 17 comprises two straight side bars 18 extending laterally and arranged between the upper and lower flat plates 15, 16 respectively at front and rear opposite side edges thereof, an intermediate bar 19 extending laterally and disposed between and spaced apart from the two side bars 18, two heat transfer area increasing portions 21 each interconnecting the intermediate bar 19 and each of the side bars 18 integrally therewith and positioned at an intermediate portion of the height of the bars, and end bars 22 extending forwardly or rearwardly inward from right ends of the respective side bars 18 integrally therewith and having respective inner ends butting on and brazed to front and rear opposite side faces of a right end of the intermediate bar 19. The intermediate bar 19 has a left end portion cut out therefrom, the two heat transfer area increasing portions 21 have respective right end portions cut out therefrom, and each of the flat plates 15, 16 has holes 13, 14 formed in a right end portion thereof respectively on front and rear opposite sides of the intermediate bar 19. Each of the two flat plates 15, 16 is bent at a left end portion thereof toward the other flat plate, and the bent portions 15a, 16a are lapped over and brazed to each other.