A power system operation assisting device configured to be coupled to a power system including a plurality of a generation sources characterised by comprising: i) a multi-signal frequency identification portion being configured to a) read measurement data and a frequency identification parameter; b) complement missing values in the read measurement data; c) generate measurement data groups generated based on a correlation of the measurement data by using the identification parameter including a correlation coefficient threshold value; d) to calculate a frequency component for each measurement data group by using at least one of Prony analysis, Fourier analysis, and wavelet analysis being included in the identification parameter; e) screen major frequency components of the said calculated frequency components of each group to determine an oscillation frequency by using the identification parameter including a threshold value of screening; ii) a generation source candidate calculation portion that is configured to determine among the plurality of generation sources one generation source candidate (D7, S2) of unstable oscillation occurring in the power system by using the calculated oscillation frequency of item e); iii) a generation source assurance test portion that is configured to calculate an assurance test result (D8, S2) concerning the generation source candidate, wherein the assurance test result is an estimation of the degree suppression of unstable vibration if an operation is performed to suppress instability by an operator; and iv) a display portion that is configured to display the oscillation frequency (D6, S4), the generation source candidate, and the assurance test result. i) a multi-signal frequency identification portion being configured to a) read measurement data and a frequency identification parameter; b) complement missing values in the read measurement data; c) generate measurement data groups generated based on a correlation of the measurement data by using the identification parameter including a correlation coefficient threshold value; d) to calculate a frequency component for each measurement data group by using at least one of Prony analysis, Fourier analysis, and wavelet analysis being included in the identification parameter; e) screen major frequency components of the said calculated frequency components of each group to determine an oscillation frequency by using the identification parameter including a threshold value of screening; a) read measurement data and a frequency identification parameter; b) complement missing values in the read measurement data; c) generate measurement data groups generated based on a correlation of the measurement data by using the identification parameter including a correlation coefficient threshold value; d) to calculate a frequency component for each measurement data group by using at least one of Prony analysis, Fourier analysis, and wavelet analysis being included in the identification parameter; e) screen major frequency components of the said calculated frequency components of each group to determine an oscillation frequency by using the identification parameter including a threshold value of screening; ii) a generation source candidate calculation portion that is configured to determine among the plurality of generation sources one generation source candidate (D7, S2) of unstable oscillation occurring in the power system by using the calculated oscillation frequency of item e); iii) a generation source assurance test portion that is configured to calculate an assurance test result (D8, S2) concerning the generation source candidate, wherein the assurance test result is an estimation of the degree suppression of unstable vibration if an operation is performed to suppress instability by an operator; and iv) a display portion that is configured to display the oscillation frequency (D6, S4), the generation source candidate, and the assurance test result.