PURPOSE: To provide a controller of a reactive power compensating equipment which can control a voltage fluctuation range both at a small voltage fluctuation and at a large voltage fluctuation such as the disturbance of a power system.

CONSTITUTION: In this system, an arithmetic circuit 7 which squares a voltage deviation V of the system voltage V from a specified reference voltage Vref is installed and outputs the value to a controlling circuit 6 of a specified control gain K. In parallel with the arithmetic circuit 7, a sign judging circuit 8 which judges a sign of the voltage deviation V input to the arithmetic circuit 7 is connected and adds a minus sign to the square signal of the arithmetic circuit 7 when the voltage deviation V is a negative value. By squaring the voltage deviation V, an apparent slope reactance XSL=1/K becomes as large as the conventional value at a small voltage fluctuation while it becomes remarkably small compared with the conventional value at a large voltage fluctuation caused by the occurrence of a disturbance, etc. Especially, the voltage fluctuation range of a power system at a large voltage fluctuation can be held down small and a reactive power compensating equipment 3 can be controlled under such a condition.