A static var compensator which includes a transformer with one end connected to an electric power system and a series circuit constituted by a reactor and a thyristor bulb connected between the other end of the transformer and ground. A voltage detection device detects the voltage of the electric power system and the output of the voltage detection device is sample an even number of times for every cycle of the voltage. A register is provided for storing sampled values an adder is provided for adding absolute values of the sampled values stored in the register. A firing angle of the thyristor bulb is controlled by a control device in accordance with a deviation of an output of the adder from an output of a voltage setting device. A deviation deriving device obtains a deviation of an output of the control device from a predetermined value corresponding to a present var, and an output of the deviation deriving devices is fed back to an input side of the control device.