In a surgical treatment instrument, a probe extending in a longitudinal axial direction is inserted through a sheath, and each of sheath components, in the sheath, is divided from a neighboring sheath component at a dividing plane along the longitudinal axial direction. A flow of liquid to a proximal side in an inside of the sheath is prevented by a first seal portion. In a divided part between each of the sheath components and the neighboring sheath component, liquid-tight is kept by the second seal portion, and thereby a flow of the liquid to the proximal side is prevented.