Abstract: A heat exchanger of a construction having a plurality of plates disposed in mutual confrontation at a predetermined spaced interval among them to separate two fluids to be subjected to heat-exchange; a fin disposed in the space interval among the mutually opposed plates to form a plurality of parallel flow paths for controlling flow of the two fluids in the spaced interval, the spaced interval formed by the plates being in a plurality of stacked layers, and the portion where the fin is present and the empty space where no fin is present being so disposed in the plurality of space intervals in layer form that they may be staggered in the direction of stacking the plates; and a control member provided in each of the space intervals in layer form to separate and alternately lead into each space interval the primary fluid and the secondary fluid so as to effect the heat exchanging operation between the primary fluid and the secondary fluid in the course of their passage through the spaced interval in layer, while