The present invention addresses the conventional problem of not assuming the time-series output change of a renewable energy and therefore possibly taking an inadequate voltage stability improvement countermeasure. According to the present invention, a voltage stability monitoring device (10) for monitoring the voltage stability of a system is characterized by comprising: an assumed change scenario calculation unit (31) for predicting the output variation of a renewable energy connected to said system and calculating an assumed change time-series power flow state on the basis of said output variation; a time-series voltage stability calculation unit (32) for calculating a node voltage change with respect to demand at each node of said system on the basis of said power flow state calculated by said assumed change scenario calculation unit and calculating a voltage stability indicating the relationship curve between said node voltage and reactive power; and a voltage stability improvement countermeasure search unit (34) for determining a controlled device on which an assumed control is to be performed, according to said voltage stability calculated by said time-series voltage stability calculation unit.