An energy surgical instrument includes a first grasping piece, a second grasping piece which is allowed to grasp a blood vessel, an energy application unit provided in at least one of the first grasping piece and the second grasping piece, which applies treatment energy to the blood vessel grasped, a setting unit which sets information regarding a wall thickness of the blood vessel, and a grasping force adjustment unit which adjusts a grasping force to a first grasping force when the wall thickness set by the setting unit is larger than a predetermined thickness and adjusting the grasping force to a second grasping force, which is smaller than the first grasping force, when the wall thickness set by the setting unit is equal to or smaller than the predetermined thickness.
