Abstract: The present invention relates to an additional air supply duct which is additionally installed at the inlet duct connected to an exhaust duct of a gas turbine of a combined-cycle power generation apparatus, for providing the air required for additional combustion in the duct burner of a heat recovery steam generator. The air supply duct of a heat recovery steam generator according to the present invention diverges to left and right at the upper side of the real portion of the inlet duct connected to the exhaust duct of the gas turbine, and the terminal parts thereof, facing each other in a staggered manner, are respectively installed through the left and right sides of the inlet duct. Further, the penetration depth into the inlet duct is set to one eighth of the width of the inlet duct. In this way, uniform distribution of oxygen concentration can be formed at the end face of the entrance of the duct burner, thereby simultaneously achieving flame stability and complete combustion.