A power system stability monitoring device comprising: a simple stability calculation unit that calculates the angular velocity and internal phase angle of a generator in each fault case based on fault conditions and the system configuration of the power system; a severity indicator calculation unit that calculates a severity indicator that represents the degree of system instability based on the angular velocity and internal phase angle; a PCS power supply severe parallel-off condition extracting unit that calculates a PCS power supply parallel-off severity indicator based on a parallel-off sensitivity indicator representing a degree of influence of PCS power supply parallel-off on the degree of instability and the severity indicator, and that extracts fault cases where the parallel-off severity indicator exceeds a prescribed threshold; and a detailed stability calculation unit that performs a detailed stability calculation for each extracted fault case based on the severity indicator or parallel-off severity indicator.