A surgical system (1; 1B; 1C) comprising: a treatment portion configured to treat living body tissue targeted for treatment; an ultrasound supplying section configured to supply ultrasound to the treatment portion via ultrasound transducers which produce ultrasound vibration; a radio-frequency electrical power supplying section configured to supply, to the treatment portion, radio-frequency electrical power having an output wave-form defined by a base frequency and a repetition frequency; a detecting section (57a, 57b) configured to detect voltage and current in a case where the radio-frequency electrical power is supplied to the living body tissue via the treatment portion; a judging section configured to compare an impedance value calculated based on a detection result from the detecting section (57a, 57b) with a threshold; and a controlling section configured to control radio-frequency electrical power outputted from the radio-frequency electrical power supplying section to the treatment portion, characterized in that the judging section is adapted to judge whether or not the calculated impedance value falls within a fatty-tissue impedance range, and the controlling section is adapted to control, when the ultrasound supplying section and the radio-frequency electrical power supplying section simultaneously output ultrasound vibration and radio-frequency electrical power to the treatment portion, according to the judgment result in the judging section, at least one of a voltage root-mean-square value and a crest factor, which is calculated by dividing a voltage peak value by the voltage root-mean-square value, so that the voltage peak value at peaks in the output wave-form of the radio-frequency electrical power does not exceed a fixed value. a treatment portion configured to treat living body tissue targeted for treatment; an ultrasound supplying section configured to supply ultrasound to the treatment portion via ultrasound transducers which produce ultrasound vibration; a radio-frequency electrical power supplying section configured to supply, to the treatment portion, radio-frequency electrical power having an output wave-form defined by a base frequency and a repetition frequency; a detecting section (57a, 57b) configured to detect voltage and current in a case where the radio-frequency electrical power is supplied to the living body tissue via the treatment portion; a judging section configured to compare an impedance value calculated based on a detection result from the detecting section (57a, 57b) with a threshold; and a controlling section configured to control radio-frequency electrical power outputted from the radio-frequency electrical power supplying section to the treatment portion, characterized in that the judging section is adapted to judge whether or not the calculated impedance value falls within a fatty-tissue impedance range, and the controlling section is adapted to control, when the ultrasound supplying section and the radio-frequency electrical power supplying section simultaneously output ultrasound vibration and radio-frequency electrical power to the treatment portion, according to the judgment result in the judging section, at least one of a voltage root-mean-square value and a crest factor, which is calculated by dividing a voltage peak value by the voltage root-mean-square value, so that the voltage peak value at peaks in the output wave-form of the radio-frequency electrical power does not exceed a fixed value.