[Problem] To provide a control method and device for a turbine generator, which can reduce generated power without wasting fuel consumption of a diesel engine when the demand power decreases and thereby the generated power is brought into an excess state. [Solution] A turbine generator configured by joining a power turbine (7) which is driven with exhaust energy of exhaust gas emitted from a diesel engine (3) as a power source and a steam turbine (9) in series, wherein the diesel engine (3) is provided with an exhaust turbosupercharger (5), extracts part of exhaust gas before the exhaust gas is supplied to the exhaust turbosupercharger (5), supplies the extracted exhaust gas to the power turbine (7) after adjusting the supply amount thereof, supplies steam generated by an economizer (11) to the steam turbine (9) after adjusting the supply amount thereof, and when the demand power decreases and thereby the generated power is brought into an excess state, decreases the supply amount of the exhaust gas to the power turbine (7) to minimize the output of the power turbine (7), and thereafter decreases the supply amount of the steam to the steam turbine (9).