In order to detect the direction of rotation with high accuracy even when an existing endoscope is used irrespective of the orientation of the endoscope, an endoscope system (1) is provided with: an endoscope (6) that has an image-acquisition optical system at a distal end of an elongated insertion portion (5)； a trocar (3) that is attached to a body surface (A) of a patient so as to penetrate the body surface (A) and that has a through-hole (2) though which the insertion portion (5) is made to pass； a rotation sensor (4) that is provided on the trocar (3) and that detects an amount of rotation of the insertion portion (5) about a longitudinal axis, the insertion portion (5) being located in the through-hole (2) in a passing-through state； a sign that is disposed at a position where it can be imaged by the image-acquisition optical system of the endoscope (6) when the insertion portion (5) is inserted into the trocar (3) and with which the position in a circumferential direction of the through-hole (2) can be identified； and an initial-angle detecting unit (7) that detects a relative angle between the insertion portion (5) and the through-hole (2) at the time of insertion on the basis of the sign in an image acquired by the image-acquisition optical system.
