Recently, the number of distributed generation (DG) that connects to a distribution system has been increasing because of reduction of the environment load. However, the harmonic troubles in a distribution system are apprehended accompanying the increase of connection of DGs through the power conditioner system (PCS) (i.e. interconnected inverter of DG) and the spread of the power electronics equipments. In this paper, the authors propose a restraint method of voltage total harmonic distortion (THD) in a whole distribution system by active filter (AF) operation of plural DGs with PCS. Moreover, the authors propose a determination method of the optimal gain of AF operation so as to minimize the maximum value of voltage THD in a distribution system by a real-time feedback control with measured data from the plural information technology (IT) switches with a built-in sensor installed in the distribution system. In order to verify the validity of the proposed method, the numerical calculations are carried out by using an analytical model of distribution system to which DGs with PCS are interconnected.