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.