A waste heat recovery boiler for generating steam by utilizing a flue gas discharged from a gas turbine as a heat source comprises an economizer for preheating feedwater by the flue gas, an evaporator for evaporating the preheated feedwater, and a superheater for generating a steam for driving a steam prime mover by heating the evaporated feedwater. The economizer, the evaporator and the superheater are successively arranged in the direction from a downstream side to an upstream side of flue gas passage. The evaporator itself is divided into two sections and an apparatus for removing NO.sub.x is provided in a space between the divided two sections of the evaporator in the flue gas passage to remove NO.sub.x from the flue gas. An NO.sub.x concentration of a flue gas discharged from a combined cycle power plant including the gas turbine can be stably abated at any of full and partial loads.