A leakage current calculation device comprising: a voltage measuring part that measures at least one of line voltages on an input side of an inverter with a three-phase AC commercial power system , in which any of three different phases is grounded, as a power source; a voltage-to-ground calculating part that calculates a voltage-to-ground (Vs∠180°) between a neutral point of the commercial power system and a ground (E) from the at least one line voltage measured by the voltage measuring part ; a leakage current measuring part that measures a leakage current flowing between a load connected to an output side of the inverter and the ground (E); characterized by : a frequency component extracting part that extracts a leakage current component (I0_sys) synchronized with a system frequency of the commercial power system from the measured leakage current; a phase difference determining part that determines a phase difference (θ) between the calculated voltage-to-ground (Vs∠180°) and the extracted leakage current component (I0_sys); and a leakage current calculating part that calculates an insulation resistance-to-ground leakage current (Iu, Iv, Iw) corresponding to the system frequency, which flows through an insulation resistance-to-ground (Ru, Rv, Rw) of the load between the load and the ground (E) on the basis of a center of a circle which is described by specific points (P1, P2, P3, P4) which are specified by the leakage current component (I0_sys) extracted and the phase difference (θ) determined at three or more different time points. a voltage measuring part that measures at least one of line voltages on an input side of an inverter with a three-phase AC commercial power system , in which any of three different phases is grounded, as a power source; a voltage-to-ground calculating part that calculates a voltage-to-ground (Vs∠180°) between a neutral point of the commercial power system and a ground (E) from the at least one line voltage measured by the voltage measuring part ; a leakage current measuring part that measures a leakage current flowing between a load connected to an output side of the inverter and the ground (E); characterized by : a frequency component extracting part that extracts a leakage current component (I0_sys) synchronized with a system frequency of the commercial power system from the measured leakage current; a phase difference determining part that determines a phase difference (θ) between the calculated voltage-to-ground (Vs∠180°) and the extracted leakage current component (I0_sys); and a leakage current calculating part that calculates an insulation resistance-to-ground leakage current (Iu, Iv, Iw) corresponding to the system frequency, which flows through an insulation resistance-to-ground (Ru, Rv, Rw) of the load between the load and the ground (E) on the basis of a center of a circle which is described by specific points (P1, P2, P3, P4) which are specified by the leakage current component (I0_sys) extracted and the phase difference (θ) determined at three or more different time points. a frequency component extracting part that extracts a leakage current component (I0_sys) synchronized with a system frequency of the commercial power system from the measured leakage current; a phase difference determining part that determines a phase difference (θ) between the calculated voltage-to-ground (Vs∠180°) and the extracted leakage current component (I0_sys); and a leakage current calculating part that calculates an insulation resistance-to-ground leakage current (Iu, Iv, Iw) corresponding to the system frequency, which flows through an insulation resistance-to-ground (Ru, Rv, Rw) of the load between the load and the ground (E) on the basis of a center of a circle which is described by specific points (P1, P2, P3, P4) which are specified by the leakage current component (I0_sys) extracted and the phase difference (θ) determined at three or more different time points. <EOS>