Provided is a plasma reactor including a reactor body having a gas inlet at a side thereof, a gas outlet at another side thereof, and an annular loop space therein, a magnetic core provided in a shape surrounding a portion of the reactor body, and wound with a primary coil for receiving power from a power unit, so as to generate plasma by exciting a gas in the annular loop space, and a control unit for determining whether the plasma is in an off state, by comparing, to a reference value, an electrical parameter related to an output of the primary coil.