PURPOSE: To enable detection of a reactive power even when a load on a system is of small capacity, by adding a reactive power energy equivalent to the minimum unit of the detection sensitivity of the reactive power at a no-load point on a detection circuit.

CONSTITUTION: The output of a transformer 3 detecting a voltage from a power source 1 is shifted to an arbitrary phase angle by a phase setting unit 5 and inputted in a reactive power detector 6 together with a load current detected by a current transformer 4. After a bias in an amount matched with the minimum detection sensitivity of the reactive power in a given system is added to the output of the reactive power detector 6, this output is inputted to comparators 9 and 10 via an amplifier 8. Then, output relays 11 and 12 are operated by the outputs of a leading reactive power detector 9 and a lagging reactive power detector 10 and thereby ON-OFF operations are conducted. By this constitution, a load becomes small and the reactive power can be detected even when the load comes near to a zero point, whereby a precise control to fix a power factor can be performed.