When a voltage-fluctuation suppression device suppresses, for input/output of connected power between a first power grid, or a commercial power grid, and a second power grid including a power generation device and grid-connected to the first power grid, voltage fluctuations in the first power grid, a control unit includes a voltage allowable range computing unit, controls a ratio between active and reactive power output by the voltage-fluctuation suppression device to be equal to a ratio between active and reactive power of power flowing between the first and second power grids, and controls the ratio between the active and reactive power output by the voltage-fluctuation suppression device to be equal to the ratio between the active and reactive power of power flowing between the first and second power grids.