PURPOSE: To allow positive detection of an overexcited operating state of a generator, a transformer, etc., by resetting the amount operated at an overexcitation operating section when the decision output from an undervoltage detecting section sustains for a predetermined time or longer.

CONSTITUTION: An overexcitation detecting/operating section 12 operates an amount corresponding to the area of AC voltage waveform using the value of AC voltage waveform sampled during the half wave interval thereof or an integer times thereof. An undervoltage detecting section 15 makes a decision whether the sampled value is lower than a preset second constant. When the decision output from the undervoltage detecting section 15 sustains for a predetermined time or longer, a time limit counter 16 resets the amount operated at the operating section 12. This constitution allows positive detection of the overexcited operating state of a generator, a transformer, etc., while preventing erroneous decision due to lowering of operational accuracy in low frequency region caused by the error at the input parts including the A/D converting part.