The invention relates to a high-frequency surgical apparatus in which, for the purpose of cutting and/or coagulating biological tissue by means of high-frequency current, a high-frequency generator having a first electrode and a second electrode forms a high-frequency circuit through the tissue being treated, with an electric arc being formed, said apparatus having a measuring device to detect DC voltage components forming in the high frequency circuit when the arc is formed, said DC voltage components being usable for controlling the high-frequency generator via a controlling device. The invention is characterized in that the measuring device has an arc-decoupling circuit which separates the DC voltage components in the positive half-cycle of the high-frequency AC voltage from the DC voltage components in the negative half-cycle of the high-frequency AC voltage and makes available at least one of the separated DC voltage components as a signal for subsequent processing in the controlling device. The invention also relates to a corresponding method of running such apparatus.