For the conventional high frequency(HF) surgical equipment, the contact impedance between the two back electrodes when they are gluing with skin is 50-200 Omega. If the contact impedance between the two electrodes is either too large or too small, an alarm is needed. This detector is designed not only to detect the contact condition between skin and the back electrodes, but also to monitor and alarm the dynamic change of the contact impedance. It effectively prevents the potential damage to medical staff when the back electrodes of the double-loop HF surgical equipment is moved during operation. Thus, this design enhances the safety of HF surgical equipment in clinical use.