A gasification unit provided with a gasification furnace 23 to produce fuel gas production is provided. A combined power generation unit 25 which generates power by rotating a gas turbine and a steam turbine using fuel gas produced in the gasification unit is provided. The combined power generation unit 25 is made operable while fuel change over between fuel gas and an auxiliary fuel as the fuel. A control system is provided in which the degree of opening of a control valve 37 for a flare stack 28 provided in a fuel gas feed line is controlled depending upon the pressure of the fuel gas from the gasification unit when fuel change over from the fuel gas to the auxiliary fuel so as to allow the fuel gas supplied to the combined power generation unit 25 to gradually flare from a flare stack 28 until a flared status that total amount of the fuel gas is reached.