A treatment system comprises a treatment tool and an energy source apparatus is used to electrically communicate with the treatment tool. The energy output source is configured to output high-frequency electric power to the bipolar electrodes so as to cause a high-frequency current to flow through a treatment target between the bipolar electrodes. A processor is used to control the energy output source. The processor performs an output control process on the heater electric power so that the heater reaches a target temperature and maintains at the target temperature. The processor detects a parameter related to the heater from the heater in the output control process based on the target temperature. The processor sets a threshold value used to determine to stop or to lower the output to the bipolar electrodes while the treatment target is being modified by the high-frequency current applied thereto based on the detected parameter.