PURPOSE: To recognize the single driving of a private AC generator accurately by a method wherein the private AC generator is judged to be in the single driving when the fluctuation of the reactive power outputted by the private AC generator is reduced by a value not less than a predetermined value.

CONSTITUTION: The output signals of the differential function 3 of a signal line 4a are sampled by a gate function 4c every predetermined phase of voltage signals generated by the voltage variable signal generating function 5 of a signal line 4b. The sampled output signals are supplied to an averaging function 4d and an average value obtained by the averaging function 4d is supplied to a memory function 4f and a single driving judging function(SD) 4g. The sampled signal stored in the memory function 4f is multiplied by a factor in a factor arithmetic function 4h after it is stored for a time of one average operation of the averaging function 4d and is supplied to the SD 4g. That is, if the output value of the averaging function 4d this time is smaller than the value obtained by multiplying the previous output value of the averaging function 4d by the factor, the SD 4g judges that a private AC generator is in the single driving and a single driving signal is supplied to a single driving alarm output function 17 from a single driving judgement signal output line 4i.