Electrosurgical treatment system includes a treatment instrument, the high frequency power generator for generating a high-frequency output for treating biological tissue, and an output unit for supplying the high-frequency output to the treatment instrument, the high frequency output at the output section a detector for detecting a voltage and current, the phase difference detecting unit for obtaining a phase difference between voltage and current the detection unit detects, by controlling the high frequency power generator, the voltage of the high-frequency output at the output unit and the first phase of applying a high-frequency output in the living tissue while changing, intended to run and the second phase of constant voltage control on the basis of a high-frequency output to be applied to the living tissue to the voltage of the period of the first phase there are, and a control unit for switching between said first and second phases by a transition condition based on the phase difference.電気外科処置システムは、処置具と、生体組織を処置するための高周波出力を発生する高周波電力発生部と、前記高周波出力を処置具に供給するための出力部と、前記出力部における前記高周波出力の電圧及び電流を検出する検出部と、前記検出部が検出した電圧と電流の位相差を求める位相差検出部と、高周波電力発生部を制御して、前記出力部における前記高周波出力の電圧を変化させながら前記生体組織に高周波出力を印加する第1フェーズと、前記生体組織に印加する高周波出力を前記第1フェーズの期間の電圧に基づいて定電圧制御する第2フェーズとを実行するものであって、前記位相差に基づく遷移条件によって前記第1フェーズと第2フェーズとを切換える制御部とを具備する。
