An open-type resonator, substantially, for quasi optical channels, formed by two reflectors which by means of permanent magnets are secured on adjusting devices mounted on movable supports. The supports themselves are mounted on guides with a possibility of movement with the aid of micrometer screws, the micrometer screw of the rear reflector can be rotated either manually or with an electric motor through a speed reducer having a variable reduction ratio, while the use of the guides provides for displacement of the reflectors through distances which considerably overcome the operating wavelength.