An electrosurgical treatment system includes a treatment instrument, a high-frequency-power generating section that generates a high-frequency output for treating a biological tissue, an output section that supplies the high-frequency output to the treatment instrument, a detecting section that detects a voltage and an electric current of the high-frequency output in the output section, a phase-difference detecting section that calculates a phase difference between the voltage and the electric current detected by the detecting section, and a control section that controls the high-frequency-power generating section and executes a first phase for applying the high-frequency output to the biological tissue while changing the voltage of the high-frequency output in the output section and a second phase for performing, on the basis of a voltage in a period of the first phase, constant voltage control of the high-frequency output applied to the biological tissue, the control section switching the first phase and the second phase according to a transition condition based on the phase difference.
