1. A medical apparatus comprising: a storage section configured to store three-dimensional image information in a subject, the three-dimensional image information being acquired in advance; an image pickup unit that includes an objective lens and acquires an optical image in the subject; a luminal organ extracting section configured to extract image information of a three-dimensional shape of a specific luminal organ in the subject from the three-dimensional image information; a position correspondence control section that sets correspondence between position information in the image information of the three-dimensional shape of the specific luminal organ in a first coordinate system and position information of the image pickup unit in a second coordinate system; a feature information acquisition section configured to acquire feature information in the image information of the three-dimensional shape of the specific luminal organ extracted by the luminal organ extracting section in a region where the image pickup unit is inserted based on the position information of the image pickup unit inserted into the specific luminal organ; an associated image generating section configured to generate an image in which the feature information is associated with the image information of the three-dimensional shape of the specific luminal organ; a determination section that determines whether or not position information of the image pickup unit in the first coordinate system, which is set to have correspondence and subjected to position estimation by the position correspondence control section, is consistent with a position of the image pickup unit in the specific luminal organ within a predetermined condition; and a control section configured to perform control to display, on a display section, a piece of the feature information in a vicinity of the position information of the image pickup unit in the specific luminal organ, when it is determined that the position information of the image pickup unit in the first coordinate system is not consistent within the predetermined condition by the determination section.