To detect a stand-alone operation of an inverter more accurately.

In a device for detecting a stand-alone operation, when estimating exciting current i_m of a pole-mounted transformer TR to which a distributed power source PV is interconnected, harmonic wave estimation means estimates, based on a value of V_m/f ratio that is a ratio between exciting voltage V_m of the pole-mounted transformer in a power system to which the distributed power source is interconnected and frequency f of the exciting voltage of the pole-mounted transformer, harmonic wave components (I_mLk, I_mrk) for each magnetization current I_mL and iron loss current I_mr that are two components of exciting current of the pole-mounted transformer TR. That means, the iron loss amount of exciting circuit of the pole-transformer TR is also treated as non-linear element. Then, based on the harmonic wave components (I_mLk, I_mrk) of the magnetization current and the iron loss current thus estimated, phase θ_mk of the harmonic wave component I_mk of the exciting current is calculated, and based on the phase θ_mk thus calculated, phase of harmonic wave current to be set flowing on a power system side is determined.