To provide the control method of an active filter, which even if a plurality of loads and power-factor improving capacitors are intermixed and a load current cannot be detected directly, can compensate for a harmonic wave similarly to the case that a load current can be directly detected.

In order to control an active filter provided on the bus-bar of a system in which a plurality of loads and power-factor improving capacitors are intermixed, a process in which the resultant current It of a load current and a capacitor current is calculated and separated into harmonic components Itn of respective orders; a process in which the harmonic voltage Vs of a system voltage is separated into harmonic components of respective orders which are divided by respective impedances corresponding to the orders of the harmonics of the capacitor current to calculate respective capacitor currents Iscn corresponding to the harmonics of respective orders; and a process in which results which are obtained by deducting the capacitor currents Iscn of the respective orders from the harmonic components Itn of the respective orders are synthesized, or the combined value Isch of the capacitor currents Iscn is deducted from the combined value of the harmonic components Itn of the respective orders to calculate the harmonic component ILh of the load current to be a compensation current output command value; are provided.