Synchronous measuring terminals 5 synchronously measure phasor quantity D101 of a voltage/current at respective measurement points 1 and 2. A data-set creating block 91 of a short-circuit capacity monitoring device 7 creates, for each measurement cycle Δt, a data set D102 containing n pieces of data based on the phasor quantities D101 of voltage and current. A phase correcting block 92 performs a phase correction on the phasor quantity using the data set D102 to create a data set D102′, and a backward impedance estimating block 93 estimates a backward impedance D103 using the data set D102′.