PURPOSE: To make turbine power steady and stabilize combustion reaction in an auxiliary combustion furnace by feedback-controlling the gas pressure in the inlet of a turbine in a specified value by operating the amount of auxiliary combustion.

CONSTITUTION: When the gas pressure in the inlet of a gas turbine detected with a detector is lower than a specified value, it means that the amount of auxiliary combustion in an auxiliary combustion furnace is insufficient. The first pressure controller 26 increases flow rate instruction values to flow rate controllers 15 and 18 in order to increase the amount of auxiliary combustion. The flow rate controllers 15, 18 operate the opening of control valves 14, 17 in order to increase fuel flow rate and air flow rate in pipelines 13, 16 to the flow rate instruction value. Thereby, the amount of auxiliary combustion is increased, and the gas pressure in the inlet of the turbine is increased to reach a specified value by mass effects attendant upon increase in combustion exhaust gas flow rate by auxiliary combustion and heat effects attendant upon heating system exhaust gas in a pipeline 10.