A combined cycle power plant having a gas turbine plant and a steam turbine plant combined together is disclosed. Steam is used for cooling a high-temperature portion (5) to be cooled around the combustor (4) of the gas turbine (2). The power plant has an air duct (9), connected to a steam path (7) for supplying steam for cooling the high-temperature portion (5), for cooling this portion using air if the steam for cooling is insufficient, typically when the plant is activated or stopped, and the air duct includes a device for boosting the air to have a pressure higher than that inside a combustor (4) of the power plant. Accordingly, the combustor and attached piping arrangement can be protected from burning and the power plant can be safely and reliably operated for a long time.