When distributed generators such as photovoltaic (PV) systems spread widely in distribution networks, it is important to detect islanding caused by power system fault for ensuring electrical safety. We developed the nonlinear magnetizing characteristics model of a pole transformer for analyzing islanding-prevention of PV system. Basic experiments of the islanding-detection were conducted by using the RLC resonant circuit for verification of the developed model. Analytical results by the proposed model agreed well with experimental results.