A combined cycle power generation plant generally comprises a gas turbine plant, a steam turbine plant and an exhaust heat recovery boiler, which further includes a low temperature reheat steam system for supplying a turbine exhaust steam from a high pressure turbine of the steam turbine plant to a reheater of the exhaust heat recovery boiler, a cooling steam supply system which branches from the low temperature reheat steam system and supplies the turbine exhaust steam to the high temperature section as a cooling steam, an intermediate pressure superheater steam system which is connected to the cooling steam supply system and adapted to join a steam supplied from an intermediate pressure superheater of the exhaust heat recovery boiler together with the turbine exhaust steam, a reheat steam system connecting the reheater of the exhaust heat recovery boiler and the intermediate pressure turbine, a cooling steam recovery system which recovers a steam after cooling the gas turbine high temperature section to the reheat steam system, a bypass system which branches a superheat steam pipe connecting an outlet of a first high pressure superheater of the exhaust heat recovery boiler and an inlet of a second high pressure superheater thereof, and a main steam system which supplies a steam from the bypass system to the high pressure turbine.