A surgery system comprising: an overtube that has a distal end, a proximal end, and a longitudinal axis and holds a treatment tool and an endoscope so as to be movable forward and backward in a direction of the longitudinal axis, the overtube having an endoscope holding part that has an endoscope holding surface for holding the endoscope and allowing circumferential rotation of the endoscope about a central axis of the endoscope, and a treatment tool holding part that has a treatment tool holding surface for holding the treatment tool; and the endoscope that is inserted into the overtube and has a held surface held by the endoscope holding surface, the endoscope having an orientation maintaining part (103, 104) that maintains the circumferential orientation of the endoscope even in a case where the overtube rotates around the longitudinal axis, wherein the orientation maintaining part (103, 104) has a rotational moment generation part (111, 160) that generates a rotational moment around the central axis of the endoscope in a case where the overtube has rotated around the longitudinal axis, and maintains the circumferential orientation of the endoscope using the rotational moment generated in the rotational moment generation part (111, 160), wherein the endoscope has: a rigid insertion part to be inserted into the overtube; a flexible cord part provided on a proximal end side of the insertion part; and a connecting part configured to connect the insertion part and the cord part to each other, wherein at least a portion of the connecting part is bent with respect to the insertion part, and wherein the rotational moment generation part (111, 160) includes the connecting part and the cord part. an overtube that has a distal end, a proximal end, and a longitudinal axis and holds a treatment tool and an endoscope so as to be movable forward and backward in a direction of the longitudinal axis, the overtube having an endoscope holding part that has an endoscope holding surface for holding the endoscope and allowing circumferential rotation of the endoscope about a central axis of the endoscope, and a treatment tool holding part that has a treatment tool holding surface for holding the treatment tool; and an endoscope holding part that has an endoscope holding surface for holding the endoscope and allowing circumferential rotation of the endoscope about a central axis of the endoscope, and a treatment tool holding part that has a treatment tool holding surface for holding the treatment tool; and the endoscope that is inserted into the overtube and has a held surface held by the endoscope holding surface, the endoscope having an orientation maintaining part (103, 104) that maintains the circumferential orientation of the endoscope even in a case where the overtube rotates around the longitudinal axis, wherein the orientation maintaining part (103, 104) has a rotational moment generation part (111, 160) that generates a rotational moment around the central axis of the endoscope in a case where the overtube has rotated around the longitudinal axis, and maintains the circumferential orientation of the endoscope using the rotational moment generated in the rotational moment generation part (111, 160), wherein the endoscope has: a rigid insertion part to be inserted into the overtube; a flexible cord part provided on a proximal end side of the insertion part; and a connecting part configured to connect the insertion part and the cord part to each other, wherein at least a portion of the connecting part is bent with respect to the insertion part, and wherein the rotational moment generation part (111, 160) includes the connecting part and the cord part. a rigid insertion part to be inserted into the overtube; a flexible cord part provided on a proximal end side of the insertion part; and a connecting part configured to connect the insertion part and the cord part to each other, wherein at least a portion of the connecting part is bent with respect to the insertion part, and wherein the rotational moment generation part (111, 160) includes the connecting part and the cord part.