A treatment system includes a treatment tool and an energy source apparatus. The treatment tool includes a heater and bipolar electrodes to grip a treatment target. The energy source apparatus supplies electrical energy to the treatment tool. A processor controls the output to the bipolar electrodes and the heater. The processor causes a high-frequency electric power to be output to the bipolar electrodes and detects a parameter that varies depending on tissue volume of the treatment target. The processor sets a target value related to an output control process for controlling the output to the heater. The processor controls the output to the heater so as to modify the treatment target with the heat of the heater. The processor increases the output and temperature to the heater until at least a predetermined point of time after starting the output control process for controlling the output to the heater.
