In a treatment system, an energy treatment instrument includes a pair of grasping pieces. An energy output source outputs electric energy to the energy treatment instrument, thereby applying treatment energy to a blood vessel grasped between the grasping pieces. A measurement section measures a blood pressure at a site related to the grasped blood vessel. A processor controls output of the electric energy from the energy output source based on a measurement result obtained by the measurement section, and thereby switches an actuation state of the energy treatment instrument between a first mode for coagulating the blood vessel and a second mode for coagulating the blood vessel target that is different from the first mode.