Two or more cores (2a, 2b) in each of which two more types of passage layers through which two or more fluids flow are layered alternately are welded together. The entire bottom portions of the cores (2a, 2b) are covered with a lower header tank (3), thereby making the fluids flow into the cores (2a, 2b). A dummy layer (14) through which none of the fluids flow is provided beside a weld side face of each core (2a, 2b). A weld spacer (18) is welded to the entire peripheral edge of a side plate (16) of the dummy layer (14). A through-hole (16a) for draining water in the dummy layer (14) is made near the lower end of the side plate of the dummy layer (14). Further, a liquid drain hole (20) through which water is drained is made at a lower corner of the weld spacer (18).