Abstract: Provided is a device for the microwave-sustained plasma treatment of gases, which comprises a hollow structure forming a waveguide intended to be connected to a microwave generator, and means for making the gas to be treated flow through the said structure in a region in which the amplitude of the electric field associated with the incident wave is high. The means for making the gas flow comprise a plasma torch for producing a plasma in the gas. The torch comprises an injector made of an electrically conducting material mounted on a first large face of the said structure and extending so as to project through an orifice made in a second large face opposite the said first large face. A gap for passage of the incident waves lies around the injector.