A duplex heat exchanger of the so-called stacked type has in principle a plurality of plate-shaped tubular elements (1) which are stacked side by side or one on another and a plurality of fins (2) each intervening between the adjacent tubular elements. Each tubular element is composed of flat tubular segments (3a, 4a) separated from each other and each communicating with one of bulged header portions (3b, 4b) of the tubular element, so that flow paths (3, 4) for heat exchanging media are formed through each tubular element. Two or more unit heat exchangers (X, Y) are defined integral with each other within the duplex heat exchanger, since the adjacent tubular elements (1) communicate with each other through the header portions (3b, 4b).