A treatment system 10 includes: a high-frequency power source 73 a power source for heat generation 83 one pair of jaws 36a, 36b having one pair of electric conductors 52a, 52b and a heater member 53, and a control section 94 that performs constant temperature control on the power source for heat generation 83 on the basis of temperature of the heater member 53 to acquire a power decrease pattern that is a prediction about a subsequent change state of the power for heat generation by using a parameter of power for heat generation based on an initial change state of the power for heat generation that increases after application of the thermal energy is started and decreases after a maximum value is exhibited, and performs pattern control on the power source for heat generation on the basis of the power decrease pattern after the power decrease pattern is acquired.
