In the present invention, a measured realtime accurate frequency is used to determine estimated instantaneous voltage/current time-series data for each phase component in accordance with the least-squares method. The estimated instantaneous voltage/current time-series data are used to determine effective voltage, effective current, instantaneous active electric power, instantaneous reactive electric power, effective active electric power, and effective reactive electric power of each phase component and symmetrical component. The measured AC electric quantities are applied to any type of electric power system control/protection apparatus.