An electrosurgical treatment system includes an output section that supplies a high-frequency output to a 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 detected voltage and electric current, and a control section that switches, on the basis of a change in the phase difference, a first phase for drying a biological tissue by applying the high-frequency output to the biological tissue while increasing the voltage to a second phase for coapting the biological tissue by performing, with a set value of a voltage determined according to a voltage value of the high-frequency output at an end point in time of the first phase, constant voltage control.