Abstract: To improve the resistance against external low-frequency disturbance (represented by generator output change), the power system stabilizer for a generator, for inputting a voltage regulating signal (e), as an auxiliary signal, to an automatic voltage regulator to control a generator terminal voltage at a target voltage (V.sub.o), comprises: an angular acceleration observer for calculating an estimated angular acceleration change rate (.DELTA.as) of the generator on the basis of at least one of stabilizing signal change rates (.DELTA.P and/or .DELTA..omega.) of the generator; and a power stabilizer circuit for calculating a corrected voltage regulating signal (e) applied to the automatic voltage regulator on the basis of the estimated angular acceleration change rate value (.DELTA.as) calculated by the angular acceleration observer. Further, it is preferable to provide a plurality of observers and a plurality of power stabilizer circuits, separately for each fluctuation frequency.