This power system operation assisting device is provided with: a multiple signal frequency specifying unit that uses, as inputs, measured data of an object to be measured and a frequency specifying parameter for calculating a frequency component of the measured data to calculate an oscillation frequency; a generation source candidate calculation unit that uses, as an input, the calculated oscillation frequency to calculate a generation source candidate of an unstable oscillation occurring in a power system; a generation source guarantee test unit that calculates the guarantee test result of the generation source candidate; and a display unit that displays the oscillation frequency, the generation source candidate, and the guarantee test result.