The ends of numerous tubes are connected to hollow headers so that they are in fluid communication with the headers. Slit shaped apertures are formed in the outside face of the headers in the direction of their circumference, and partitions are inserted through the apertures. Each partition is constructed out of two partition plates which fit into the header through the aperture in their superimposed position. Alternatively, the partition plates may be connected integral at one end so that their unconnected ends tightly contact the edges of the slit shaped aperture. The partitions are then brazed to the header to become integral with it.