PURPOSE: To enable detection of a momentary voltage drop in real time, by comparing an input with the results obtained by subtracting a instantaneous voltage drop detection level at a peak value point from an input waveform as reference waveform.

CONSTITUTION: A PLL circuit 22 receives an input voltage vS and feeds an output thereof as synchronous signal to a reference sine wave generation circuit 23 which generates a reference sine wave vS. The circuit 23 subtracts a momentary voltage drop detection level value vl (e.g. vl=0.1 vSO)at a peak value vP from an official valve VSO of the input voltage VS, giving a waveform. An absolute value conversion circuit 24 output an absolute value v2 of the sine wave V2 and an absolute value conversion circuit 25 outputs an absolute value vS of the voltage vS. A circuit 25 derives a difference of the absolute value (vS-2=Δv20) and an instantaneous voltage drop discriminator circuit 27 judges that a momentary voltage drop occurs if vS<v2 and generates an output U. Thus, the momentary voltage drop is discriminated in real time.