To use a catheter for exactly distinguishing a small separation causing a problem between a surgical tool and a surgical treatment part and a small separation not causing a problem by constituting a monitoring configuration part so as to deal with the temporary change of the monitoring configuration part in a spatial separation.

A detector 5 receives the sine fluctuation ultrasonic signal of reflected illumination, supplies that signal to an analyze 6 and converts it to digital indication. Through programming based on well-known techniques, the analyzer 6 compares the frequency of a reflected waveform from the detector 5 with the frequency of a wavefom used for a generator 9 and provides information on the temporary change of relative separation between an electrode 3 and the surgical treatment part. For example, a skilled person can expect the sudden repetition of fibrous contraction of the heart just under the surgical operation of the heart. In such a case, the rapid and small amplitude change is predicted in the beginning. The monitoring system of the temporary change can detect such a rapid action and suppresses the operation of the electrode 3 before enlarging a spatial breakaway.