Abstract: A waste heat boiler which comprises a water chamber and a water vapor chamber, both surrounded by an opposed pair of tube plates, respectively, and a plurality of double tubes extending across the two chambers. The inner tubes within the double tubes are adapted to pass hot waste gases therethrough, while annular spaces defined between the inner and outer tubes within the double tubes are for directing water to be heated therethrough. At least one end area of the double tubes is of triplicate structure so that thermal stresses generated from thermal expansion of the tube elements may be absorbed effectively.