According to the present invention, a sampled value of a reference voltage V.sub.p which is a voltage (in general, a relative voltage to ground, an interphase voltage or a positive sequence voltage) prior to the occurrence of a fault in an electric-power system and a sampled value of the quantity of electricity E (a voltage or current in each phase or a symmetrical component voltage or current) during a fault time period or after the recovery of a fault are used to compute a plurality of functional values capable of specifying the phase angle .theta. or the value of .theta. of said quantity of electricity E in relation to said reference voltage V.sub.p and deliver them as information data. When the relative phase angle between the quantity of electricity E during a fault or after the recovery of a fault and a voltage (reference voltage) V.sub.p prior to the occurrence of a fault is clarified in this manner, the relative phase angles of reference voltages V.sub.p among a plurality of electric stations prior to the occurrence of a fault are computed by obtaining the information of normal operating conditions of the electric power system concerning to a voltage, current, reactive power and the state of a circuit breaker and accomplishing the arithmetic operations of the information thus obtained and the constants of devices and equipment constituting the electric-power system. Therefore, the relative phase angles of the quantity of electricity E among a plurality of electric stations during a fault period or after the recovery of a fault can be clarified.