PURPOSE: To detect a step-out state without using a threshold, value by measuring power and voltage and detecting an oscillatory phenomenon in power system.

CONSTITUTION: A measuring unit 1 is used for measuring active power and effective current in linkage lines at given cyclic intervals. A step-out judgement block 2 judges a step-out state by detecting a fact that an operation point in power system is shifted from a stable region to an unstable region on the basis of a variation in active power and voltage measured every moment and a comparison between the active power measured every moment and the active power in a stationary state. In this case, a cyclic fluctuation factor in active power and a current (effective value) measured at given intervals and an actually measured value of active power are used. In this way, the step-out phenomenon can be detected without using a threshold value.