To easily perform movement such as rotation of an internal organ with low invasion.

A surgical instrument 1 includes: an instrument body 3 which is disposed rotatably around the long axis of a tubular sheath tube 2 penetrating a body surface tissue A of a patient and having an inner hole 2a opened inside and outside the patient body, inside the inner hole 2a of the sheath tube 2, and has a plurality of through holes 3a penetrating in the long axis direction; and a plurality of arms 4 each of which is inserted into each through hole 3a of the instrument body 3 position-changeably in the length direction of the through hole 3a, and can be fixed to the instrument body 3. Each arm 4 includes, at a tip disposed inside the patient body, a fixing part 4b to be projected from an opening inside the patient body, and fixed to a surface of an internal organ.