A medical instrument including a distortion detection probe disposed in an insertion portion to be inserted into the interior of an examinee provided with a plurality of FBG sensor sections that detect distortion of the insertion portion, a coordinate calculation section that calculates first three-dimensional coordinates of the respective FBG sensor sections according to a first three-dimensional coordinate system whose origin is a predetermined position based on the detection results of the FBG sensor sections, a coordinate system setting section that sets a second three-dimensional coordinate system based on the first three-dimensional coordinates of the respective FBG sensor sections, a coordinate transformation section that transforms the first three-dimensional coordinates of the respective FBG sensor sections into second three-dimensional coordinates according to the second three-dimensional coordinate system set by the coordinate system setting section and a shape display section that displays the shape of the insertion portion based on the second three-dimensional coordinates transformed by the coordinate transformation section.
