an apparatus for stabilizing turbine generator shaft torsional oscillations including : a current detector for detecting ac current of a turbine generator used in combination with a dc power - transmission system , a filter to which an output of said current detector is delivered , the filter comprising a first bandpass filter for detecting a sub synchronous component and a second bandpass filter for detecting a super synchronous component where a fundamental freuency of the generator is represented by f l and a natural frequency of the mechanical shaft system of the generator is represented by f m , a calculation circuit for applying conversion of at least one of d and q axis to respective outputs from the first and second bandpass filters using a generator phase θ g to take out at least one of δi d and δi q of the sub and super synchronous components , a third bandpass filter to which an output from the calculation circuit is fed , the third bandpass filter for taking out a stabilization signal therefrom , and a circuit for controlling a converter for delivering an output of the generator to the dc power - transmission system on the basis of the stabilization signal .