A high frequency surgical apparatus for the thermal coagulation of biologic tissues is described, in which for monitoring the fluctuations in amplitude of the high frequency current during each coagulation process a current monitor (25) is provided, which by means of a current-to-voltage coverter generates an electrical voltage proportional to the amplitude fluctuations of the high frequency current, from which voltage, by means of a first detector, a first direct voltage proportional to the amplitude fluctuations is formed and, by means of a second detector acting as a peak value detector, a second direct voltage is formed that rises in proportion to the amplitude of the high frequency current. The first direct voltage (U.sub.a) and the second direct voltage (kU.sub.b), which is divided downward by an adjustable factor by means of a voltage divider, are supplied to a voltage comparator, the output signal of which sets a bistable circuit as soon as the first direct voltage (U.sub.a) becomes lower than the downwardly divided second direct voltage (kU.sub.b). The output signal of the bistable circuit switches the high frequency current off until the high frequency current is switched back on again by actuation of a manual switch and/or by means of an automatic switch. Instead of or in addition to this current monitor (25), and electric arc monitor (26) for monitoring the generation of anharmonic frequencies of the base frequency of the frequency generator can be connected to the output of the surgical apparatus, the arc monitor including a filter which passes at least one of the anharmonic frequencies, generated by the electric arc, of the base frequency of the high frequency generator. The output signal of the filter is supplied to a voltage comparator, the output signal of which resets a bistable circuit such that its output signal switches off the high frequency current.