The amount of distributed generation (DG) such as photovoltaic systems and wind power generator systems installed in distribution systems has been increasing because of reduction of the effects on the environment. However, harmonic problems in the distribution system are a concern in the context of increased connection of DGs through inverters and the spread of power electronics equipment. This paper proposes a method for the restraint of voltage total harmonic distortion (THD) in a whole distribution network by active filter (AF) operation of multiple power conditioner systems (PCSs). It also proposes a method for determination of the optimal gain of AF operation so as to minimize the maximum value of voltage THD in the distribution network by real‐time feedback control with measured data from information technology (IT) switches. In order to verify the validity of the proposed method, numerical calculations are carried out using an analytical model of a distribution network of interconnected DGs with PCSs. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 182(3): 19–29, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22354