First heat-transfer plates S1 and second heat-transfer 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-transfer plates S1 and second heat-transfer plates S2 are jointed 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-transfer plates S1 and second heat-transfer plates S2 in the radial direction, thereby avoiding a decrease in heat exchanging efficiency and generation of unwanted thermal stress.