Abstract: A waste heat recovery boiler apparatus for generating steam by utilizing the heat of exhaust gases of a gas turbine machine, comprising a passage connected at one end thereof to the gas turbine machine, said exhaust gases passing through said passage, a fuel economizer arranged in the passage for preheating feed water, an evaporator arranged in said passage and disposed on the upstream side of said fuel economizer for vaporizing the preheated feed water, a superheater arranged in said passage and disposed on the upstream side of said evaporator for superheating the vaporized feed water to generate superheated steam, and a denitration device arranged in said passage and disposed in a zone in the passage for removing oxides of nitrogen from the exhaust gases. The exhaust gases in such zone have a temperature which is in the range between 300.degree. and 400.degree.