In the present invention, with the purpose of accurately detecting, even when using an existing endoscope, the direction of rotation of an endoscope, irrespective of the posture thereof, an endoscope system (1) is equipped with: an endoscope (6) having an imaging optical system provided at the distal end of a long, thin insertion part (5); a trocar (3) attached to a patient's bodily surface (A) by being passed through the bodily surface (A), and having a passage hole (2) through which the insertion part (5) is passed; a rotation sensor (4) provided to the trocar (3) and detecting the amount of rotation of the insertion part (5) about the lengthwise axis thereof, with the insertion part being disposed in a state of being passed through the passage hole (2); and an initial angle detection unit (7) disposed at a location that can be imaged by the imaging optical system of the endoscope (6) as inserted into the trocar (3), and detecting the relative angle of the insertion part (5) and the passage hole (2) at the time of insertion, on the basis of a display by which the circumferential position of the passage hole (2) can be identified, and a display within an image acquired by the imaging optical system.