Even when the optical axis is offset in a rigid endoscope, the actual optical axis position is measured in advance, and surgery navigation can be performed by using the actual optical axis position. A surgery assistance system comprising 3-dimensional shape measurement means for measuring a 3-dimensional surface shape of a patient and a rigid endoscope (11) to be inserted into the body of the patient wherein the rigid endoscope has a position-orientation detection marker (12) measurable by the 3-dimensional shape measurement means 3-dimensional tomographical data of the patient imaged in advance and a position relationship relating to the actual optical axis position of the rigid endoscope measured in advance are stored in the computation means 3-dimensional tomographical data and the 3-dinensional surface shape of the patient measured by the 3-dimensional shape measurement means are aligned the position, orientation, and optical axis of the rigid endoscope are computed on the basis of the position-orientation detection marker measured by the 3-dimensional shape measurement means and the stored actual optical axis position of the rigid endoscope and the aligned 3-dimensional tomographical data, the optical axis of the rigid endoscope, and the intersection of the optical axis and the tissue wall are displayed.
