An electrosurgical treatment system comprising: a high-frequency-power generating section configured to generate a high-frequency output for treating a biological tissue; an output section configured to supply the high-frequency output to a treatment instrument that is configured to apply the high-frequency output to the biological tissue; a detecting section configured to detect a voltage and an electric current of the high-frequency output in the output section; a phase-difference detecting section configured to calculate a phase difference between the voltage and the electric current detected by the detecting section; and a control section configured to control the high-frequency-power generating section and switch, on the basis of a change in the phase difference, a first phase for drying the biological tissue by applying the high-frequency output in the output section to the biological tissue while increasing the voltage with respect to a voltage at a start time to a second phase for coapting the biological tissue by performing, with a set value of a voltage determined according to a voltage value at an end point in time of the first phase, constant voltage control of the high-frequency output applied to the biological tissue, wherein the control section is configured to switch the first phase to the second phase on the basis of a positive phase difference starting to increase or a negative phase difference starting to decrease, and wherein, after shifting from the first phase to the second phase, the control section is configured to switch the second phase to the first phase on the basis of the positive phase difference decreasing or the negative phase difference increasing, characterized in that in the second phase, the control section is configured to perform the constant voltage control in which a voltage value of the high-frequency output at an end point in time of the first phase is maintained. a high-frequency-power generating section configured to generate a high-frequency output for treating a biological tissue; an output section configured to supply the high-frequency output to a treatment instrument that is configured to apply the high-frequency output to the biological tissue; a detecting section configured to detect a voltage and an electric current of the high-frequency output in the output section; a phase-difference detecting section configured to calculate a phase difference between the voltage and the electric current detected by the detecting section; and a control section configured to control the high-frequency-power generating section and switch, on the basis of a change in the phase difference, a first phase for drying the biological tissue by applying the high-frequency output in the output section to the biological tissue while increasing the voltage with respect to a voltage at a start time to a second phase for coapting the biological tissue by performing, with a set value of a voltage determined according to a voltage value at an end point in time of the first phase, constant voltage control of the high-frequency output applied to the biological tissue, wherein the control section is configured to switch the first phase to the second phase on the basis of a positive phase difference starting to increase or a negative phase difference starting to decrease, and wherein, after shifting from the first phase to the second phase, the control section is configured to switch the second phase to the first phase on the basis of the positive phase difference decreasing or the negative phase difference increasing, characterized in that in the second phase, the control section is configured to perform the constant voltage control in which a voltage value of the high-frequency output at an end point in time of the first phase is maintained.