To detect an effective current Igr with high precision, free of influence from harmonics contained in the voltage/current of a circuit.

A zero-phase current I0(t) and a voltage to ground V(t) of a grounding wire of a low-voltage circuit are detected, and the zero-phase current and the voltage to ground are converted into instantaneous value of digital data. Fundamental frequency components of the zero-phase current and the voltage to ground are extracted, by subjected the fundamental frequency components of the circuit from the discrete value data corresponding to one cycle of the instantaneous value data digital Fourier transformation. The effective current Igr is determined by inner product calculations of vectors from the fundamental wave Fourier coefficients of the fundamental frequency components. The insulation condition is decided from the magnitude of the effective current Igr. When the voltage to ground is in a lower level, insulation condition monitoring by the effective current is switched to insulation condition monitoring by the zero-phase current. The insulation condition is decided, by providing detection characteristics with fan-shaped phase characteristics.