A voltage stability monitoring device for monitoring a voltage stability of an electrical power system, the voltage stability monitoring device is configured to receive system measurement data from the electrical power system and to estimate the voltage stability by using a voltage stability curve representing the voltage stability in the electrical power system, characterized in comprising: - a voltage stability limit prediction unit configured to execute voltage stability limit prediction calculation, comprising a provisional determination for a nose end of a voltage stability curve, using the system measurement data, system facility data and calculation setting data of an input database, and to output the result of the calculation to calculation result data, wherein the system measurement data preferably comprises an effective power, a reactive power, a power factor (φ), a voltage, a voltage phase angle (δ) of a node, a branch, a power source, a load, or a control device connected to the electrical power system for measurement, - a voltage stability calculation condition determination unit configured to determine voltage stability calculation conditions by using a result of the voltage stability limit prediction calculation and comprising a determination of a value of a geometrical parameter, and to output the calculation result to the calculation result data, - a voltage stability curve calculation unit configured to calculate the voltage stability curve by using the voltage stability calculation conditions, and to output the calculation result to the calculation result data, - a voltage stability margin calculation unit configured to calculate a voltage stability margin by using the result of the voltage stability curve calculation of the calculation result data, and to output the result of the calculation to the calculation result data, wherein the voltage stability margin calculated in the voltage stability margin calculation unit is a deviation from an operational point to a limit point obtained by using the voltage stability curve calculated by the voltage stability curve calculation unit, and - a display unit for displaying one or more information in the calculation result data. - a voltage stability limit prediction unit configured to execute voltage stability limit prediction calculation, comprising a provisional determination for a nose end of a voltage stability curve, using the system measurement data, system facility data and calculation setting data of an input database, and to output the result of the calculation to calculation result data, wherein the system measurement data preferably comprises an effective power, a reactive power, a power factor (φ), a voltage, a voltage phase angle (δ) of a node, a branch, a power source, a load, or a control device connected to the electrical power system for measurement, - a voltage stability calculation condition determination unit configured to determine voltage stability calculation conditions by using a result of the voltage stability limit prediction calculation and comprising a determination of a value of a geometrical parameter, and to output the calculation result to the calculation result data, - a voltage stability curve calculation unit configured to calculate the voltage stability curve by using the voltage stability calculation conditions, and to output the calculation result to the calculation result data, - a voltage stability margin calculation unit configured to calculate a voltage stability margin by using the result of the voltage stability curve calculation of the calculation result data, and to output the result of the calculation to the calculation result data, wherein the voltage stability margin calculated in the voltage stability margin calculation unit is a deviation from an operational point to a limit point obtained by using the voltage stability curve calculated by the voltage stability curve calculation unit, and - a display unit for displaying one or more information in the calculation result data.