Abstract: A heat exchanger is formed by connecting tube-group blocks along a tube axis, where each one of tube-group blocks includes a plurality of substrates having a large number of through holes, which communicate with insides of a plurality of tubes placed between the substrates. A length of the tubes can be shortened so that the tube-group block can be formed within a predetermined size. The substrates and the tubes can be formed by injection molding or die-casting simultaneously with ease, so that the manufacturing steps of inserting the tubes and bonding the substrates can be eliminated. The heat exchanger can be available at a lower cost while it maintains excellent heat exchanging performance.