This invention is structured by:forming a core body by turning-up a strip-shaped metal plate in fanfold manner to create a large number of flat flow passages; blocking the individual flow passages at both ends thereof using the respective comb teeth of a pair of comb-state members; and fitting the casing to the core body, while providing inlet/outlet port of the fluid at a side face of the casing. The object of the invention is to establish uniform communication of the fluid in individual flat flow passages. As a means for achieving the object, a pair of header portions are provided at both end portions of the cylindrical casing, the inlet/outlet ports are provided at both edge portions of one side of the casing via a pair of small tank portions, the small tank portion at inlet side of the first fluid has a buffer plate arranged at a position closer to the outlet side of the first fluid between the core body and the inlet/outlet port, thereby allowing the first fluid to bypass the buffer plate in the small tank portion and to enter an end portion of the first flow passage from an edge opposite to the outlet.