A method for monitoring a short-circuit capacity of a power system, the method comprising: a data measuring step of measuring synchronously at multiple time points per measurement cycle, phasor quantities of a voltage and a current at each of at least two measurement points that are a first measurement point and a second measurement point on a power line of the power system, and a phasor quantity of a voltage at another measurement point for a phase correction connected to the measurement points via the power line of the power system; a data collecting step of collecting measured data measured in the data measuring step; a data-set creating step of creating a data set for each of the first measurement point, the second measurement point and the another measurement point for a phase correction, the data set containing a plural pieces of data of the multiple time points per measurement cycle based on the measured data collected in the data collecting step; a phase correcting step of correcting a phase of the data set by a process of eliminating a change in the system frequency and a process of maintaining the consistency in the phase at the multiple time points; a backward impedance estimating step of estimating a backward impedance using the data set having undergone the phase correction in the phase correcting step; and a short-circuit capacity calculating step of calculating a short-circuit capacity based on the backward impedance estimated in the backward impedance estimating step, wherein in the process of eliminating a change in the system frequency, a voltage phase of the another measurement point for the phase correction is subtracted from the voltage phase of the first measurement point at each time point of the data sets, and the voltage phase of the another measurement point for the phase correction is subtracted from the voltage phase of the second measurement point at each time point of the data sets, and in the process of maintaining the consistency in the phase at the multiple time points, a voltage phase at a certain arbitrary time and at any one of the first and second measurement points is defined as a reference phase, the reference phase is subtracted from the voltage phase of the first measurement point, and the reference phase is subtracted from the voltage phase of the second measurement point. a data measuring step of measuring synchronously at multiple time points per measurement cycle, phasor quantities of a voltage and a current at each of at least two measurement points that are a first measurement point and a second measurement point on a power line of the power system, and a phasor quantity of a voltage at another measurement point for a phase correction connected to the measurement points via the power line of the power system; a data collecting step of collecting measured data measured in the data measuring step; a data-set creating step of creating a data set for each of the first measurement point, the second measurement point and the another measurement point for a phase correction, the data set containing a plural pieces of data of the multiple time points per measurement cycle based on the measured data collected in the data collecting step; a phase correcting step of correcting a phase of the data set by a process of eliminating a change in the system frequency and a process of maintaining the consistency in the phase at the multiple time points; a backward impedance estimating step of estimating a backward impedance using the data set having undergone the phase correction in the phase correcting step; and a short-circuit capacity calculating step of calculating a short-circuit capacity based on the backward impedance estimated in the backward impedance estimating step, wherein in the process of eliminating a change in the system frequency, a voltage phase of the another measurement point for the phase correction is subtracted from the voltage phase of the first measurement point at each time point of the data sets, and the voltage phase of the another measurement point for the phase correction is subtracted from the voltage phase of the second measurement point at each time point of the data sets, and in the process of maintaining the consistency in the phase at the multiple time points, a voltage phase at a certain arbitrary time and at any one of the first and second measurement points is defined as a reference phase, the reference phase is subtracted from the voltage phase of the first measurement point, and the reference phase is subtracted from the voltage phase of the second measurement point.