In a system to which a fluctuating load is connected, compensating for fluctuation in voltage harmonics at the load connecting point and fluctuation in system current harmonics has been difficult for a power converting device connected in parallel with the load. To resolve the problem, a power converting device connected in parallel with a fluctuating load includes: a Fourier series expansion unit which executes Fourier series expansion to load current by use of a reference sine wave in sync with a system and thereby outputs Fourier coefficients; and a fundamental component calculating unit which calculates a positive phase active fundamental component of the load current from the Fourier coefficients. A current instruction of the power converting device is generated by subtracting the fundamental current from the load current. With the current instruction, the fluctuations in system current harmonics and in voltage harmonics at the connecting point can be compensated for.