In a treatment system, an energy treatment instrument includes a pair of grasping pieces closing with respect to each other. An energy output source outputs electric energy to the energy treatment instrument, thereby applying treatment energy to a treatment target grasped between the grasping pieces. A processor creates an optical flow of the treatment target observed by an observation element, and switches an actuation state of the energy treatment instrument between a first mode for treating the treatment target and a second mode for treating the treatment target that is different from the first mode, based on the optical flow.