the feedwater to be supplied to the economizer 25 of the boiler 2 the steam turbine 33 has a turbine by pass system consisting of a turbine by pass pipe 47 and a by pass valve 48 branching from the main steam pipe 31 and leading to the condenser 41 so that an excess steam can be discharged into the condenser 41 the steam turbine 33 is also provided with a system consisting of a gland seal steam pipe 35 and a gland seal steam regulator 36 through which the steam is supplied to the glands to seal them this embodiment of the invention is also characterized in that the steam pressure at the exit of the heat recovery boiler 2 can be adjusted by controlling a pressure regulating valve 62 by a pressure regulator 64 the pressure regulating valve 62 being provided to a steam blow pipe 61 which branches from the main steam pipe 31 at the exit of the heat recovery boiler 2 and leads to a blow tank 63 to ensure an effective operation it is necessary that the capacity of the regulating valve 62 be more than 2 of the rated steam capacity of the heat recovery boiler 2 but not exceeding 50 for economic discharge of steam and lower cost of equipment the capacity of the conventional pressure regulating valve is less than 1 of the rated capacity.