A treatment system 1 with which appropriate treatment is easily performed is provided. The treatment system 1 includes a treatment instrument 2 having a heating element 13 which applies thermal energy to living tissue, an element temperature measuring section 39 configured to measure an element temperature T1 of the heating element 13, a power source 30 configured to generate power, a first calculating section 33A configured to estimate a temperature difference ”T between the element temperature T1 and a temperature T2 of the living tissue using a table or an equation stored beforehand in order to estimate the temperature difference ”T based on an output value of the power source 30, a second calculating section 33B configured to estimate the tissue temperature T2 from the element temperature T1 and the temperature difference ”T estimated by the first calculating section 33B, and a control section 34 configured to control the power source 30 based on the tissue temperature T2 estimated by the second calculating section 33B.
