An endoscope system includes: an endoscope having an image-acquisition optical system at a distal end of an elongated insertion portion； a trocar attached to a body surface so as to penetrate the body surface and having a through-hole though which the portion is made to pass； a rotation sensor provided on the trocar and detecting an amount of rotation of the portion about a longitudinal axis, the portion being located in the through-hole in a passing-through state； a sign disposed at a position where it can be imaged by the image-acquisition optical system when the portion is inserted into the trocar and with which the position in a circumferential direction of the through-hole can be identified； and an detecting unit detecting a relative angle between the portion and the through-hole at the time of insertion on the basis of the sign in an image acquired by the image-acquisition optical system.
