Provided is a surgery assistance system including: a body fitting jig 1 which is to be attached to a predetermined attachment site of a body on which surgery is to be performed； and a surgery assistance device which assists surgery by displaying an augmented reality image. The body fitting jig has: a site engaging part 11 which is engageable with the predetermined attachment site； and an image-recognizable part 12 which is provided with features which can be image-recognized for the display of the augmented reality image by the surgery assistance device. The surgery assistance device has a central control unit which recognizes the body fitting jig attached to the predetermined attachment site and the image-recognizable part in a captured image of the site to be operated, estimates the position and posture of the body fitting jig in the captured image, and causes a display unit to display a first augmented reality image superimposed over the captured image, the first augmented reality image virtually representing the site to be operated in a position and posture corresponding to the estimated position and posture of the body fitting jig in the captured image.
