A control unit of a system stabilizing device uses a fluctuation detecting block (70A) to determine fluctuation components included in the active component and reactive component currents of a system current. The fluctuation detecting block (70A) is composed of a low-pass filter (71) for noise removal having first order lag characteristics with a time constant of T1, a low-pass filter (72) for setting a fluctuation detection time having first order lag characteristics with a time constant of T2, a subtracter (73) which performs subtraction between the output signals of the filter (71) and the filter (72) and outputs the difference, an amplifier (74) for amplifying the output of the subtracter (73), a rating limiter (75) for limiting the output of the amplifier, and an overcompensation inhibiting unit (200) which changes T4 to a smaller value when the rating limiter (75) performs a limiter action.