A high frequency current application apparatus for thermal sclerosing of body tissue, comprising a high frequency generator connected to at least two electrodes, one of which is to be introduced into body tissue. The apparatus includes a measuring and calculating device adapted to ascertain impedance (or ohmic resistance) between the two electrodes between which a respective therapeutic high frequency current flows. A control device is connected to the measuring and calculating device for varying the output power. During high frequency current application the measuring and calculating device forms the first derivative with respect of time of the impedance. The control device reduces output power when that derivative exceeds a positive threshold characteristic of a threat of tissue desiccation and restores output power to the original power when the first derivative exceeds in positive values a negative threshold characteristic of adequate rehydration of body tissue.