To stably monitor a separating state independently of the impedance of a patient by surely and linearly detecting extremely fine contact impedance.

When the contact impedance detecting signal of the feedback electrode and a human body detected by a contact impedance detecting signal 22 is within a normal range consisting of prescribed upper/lower limit values just after the power supply is turned on, a main control part 40 stores the impedance detecting signal as a reference signal. Then, when a difference value between the contact impedance detecting signal generated by the part 22 and the reference signal exceeds a fixed level, an abnormal state that a feedback electrode is separated from the human body is discriminated. On the other hand, the contact impedance detecting signal is at a level higher than the reference signal or/and returns to a normal state again after being discriminated to be abnormal once, the reference signal is replaced with a contact impedance detecting signal at the point of time.