An end effector grasps a treatment target between a pair of gasping pieces, and an energy output source outputs electric energy so that treatment energy is applied to the grasped treatment target. A processor switches a driving state of an actuator based on a fact that a state of the treatment target has become a predetermined state when the electric energy is output from the energy output source. As the result, a state of the end effector is switched from a first grasping state to a second grasping state, a grasping force and a grasping pressure between the first grasping piece and the second grasping piece in the second grasping state being different from those in the first grasping state.