First heat exchanger plates (S1) and second heat exchanger plates (S2) are radially arranged between a larger diameter cylindrical-shaped outer casing (6) and a smaller diameter cylindrical-shaped inner casing (7) to form combustion gas passages (4) and air passages (5) alternately in a circumferential direction, and a multiplicity of projections (22, 23) formed on both surfaces of the first heat exchanger plates (S1) and second heat exchanger plates (S2) are joined to one another at tip ends thereof. Pitches (P) between adjacent projections (22, 23) are changed in a radial direction to make the number of heat transfer units substantially constant in a radial direction to uniformize temperature distributions on the first heat exchanger plates (S1) and second heat exchanger plates (S2) in the radial direction, thereby avoiding a decrease in heat exchanging efficiency and generation of unwanted thermal stress.
