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This paper proposes a series active filter for mitigation of the 3rd-harmonic voltage in a three-phase four-wire power distribution system in a building. The active filter which consists of a single-phase inverter can suppress the harmonic voltage of the system. The active filter is characterized by acting not only as a capacitor but also as a resistor for the 3rd-harmonic components. A Hilbert transformer is applied to the controller of the active filter in order to realize the accurate 3rd-harmonic detection on a single-phase active filter. Measurement results of harmonic distortion of source voltage in a building are also shown in this paper. It is clarified in a simulation and experiment that the active filter can suppress the 3rd-harmonic voltage without increasing neutral conductor current.