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 (3, 3a, 3b) 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 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 (5, 5a, 5b) connected to an output side of the inverter (3, 3a, 3b) and the ground; 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 (5, 5a, 5b) between the load (5, 5a, 5b) and the ground on the basis of a center (74, 83, 92) of a circle (61, 73, 82, 91) 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 (3, 3a, 3b) 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 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 (5, 5a, 5b) connected to an output side of the inverter (3, 3a, 3b) and the ground; 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 (5, 5a, 5b) between the load (5, 5a, 5b) and the ground on the basis of a center (74, 83, 92) of a circle (61, 73, 82, 91) 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 (5, 5a, 5b) between the load (5, 5a, 5b) and the ground on the basis of a center (74, 83, 92) of a circle (61, 73, 82, 91) 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.