PURPOSE: To prevent the erroneous detection of route disconnection fault and top raise the reliability.

CONSTITUTION: The title detector for route disconnection fault has a first judging means for judging whether or not the absolute value P of the effective power P of a transmission line is more than a set value KPH, a second judging means for judging whether or not P is less than a set value KPL (KPH<KPL), a third judging means for judging whether or not the absolute value Idot of a current vector flowing in the transmission line is more than a set value KIB, and a fourth judging means for judging whether or mot Idot is less than a set value KIL (KIB<KIL), And, it is judged that a route disconnection fault has occurred when the absolute value P of the effective power P moved from more than KPH to less than KPL, and the absolute value Idot of the current vector moved from more than KIB to less than KIL, on the basis of judged results by the first to fourth judging means.