A treatment system includes a power source for heat generation which outputs power for heat generation, a grasping member having a heating element which applies the power for heat generation as thermal energy to a grasped living tissue and is disposed at a grasping surface, and a control section which repeats a first control mode of performing control such that the heating element reaches a first temperature and a second control mode of performing control such that the heating element becomes lower than the first temperature, and controls the power source for heat generation according to a temperature change parameter based on change in temperature of the heating element in the first control mode or the second control mode so as to end application of the thermal energy.