A combined cycle plant includes a gas turbine fuel control valve which controls supply of first fuel; a duct burner fuel control valve which controls supply of second fuel; a generator; a gas turbine, a boiler and a steam turbine. The gas turbine drives the generator by burning the first fuel supplied through the gas turbine fuel control valve. The boiler has a duct burner, heats gas exhausted from the gas turbine by burning the second fuel supplied through the duct burner fuel control valve by the duct burner, and heats water with the heated exhausted gas to generate steam. The steam turbine is driven with the steam to drive the generator. The combined cycle plant is further comprised of a controller which controls the gas turbine fuel control valve and the duct burner fuel control valve. The controller controls an open degree of the gas turbine fuel control valve such that rapid increase of the supply of the first fuel is restrained when the supply of the second fuel to the duct burner is stopped.