To provide a plasma reactor capable of compensating the output loss in a transmission line caused by reflecting wave power from a reaction container, and stably generating strong plasma.

A plasma reactor has a cylindrical reaction container to which gas for plasma is supplied; an outside electrode 2 arranged in the outer circumference of the reaction container; a metal cylinder arranged on the inside of the reaction container; and a high frequency power source for supplying high frequency power to the outside electrode 2 through a matching circuit for impedance control, and treatment such as etching or ashing is conducted to a semiconductor substrate with plasma generated. When plasma is generated, an output control means attached to the high frequency power source B detects reflecting wave power from the outside electrode 2 side with a reflecting power meter B4, and adds to output power equivalent to the reflecting wave power to an output circuit B2 with a controller B1.