1. A medical apparatus comprising: a storage device configured to store three-dimensional image information of a subject, the three-dimensional image information being acquired in advance; an image pickup device configured to acquire an optical image inside the subject; and a processor comprising hardware, wherein the processor is configured to implement: 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 configured to generate position correspondence information in an initialization state in which correspondence is set between position information of a predetermined position in the specific luminal organ in a first coordinate system and position information of a distal end portion that is set to the predetermined position in a second coordinate system, thereby setting correspondence between position information in the image information of the three-dimensional shape of the specific luminal organ in the first coordinate system and position information of the image pickup device in the second coordinate system; a feature information acquisition section configured to acquire, as feature information, position information related to a branch region where a lumen branches 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 device is inserted based on the position information of the image pickup device inserted into the specific luminal organ in the first coordinate system, the position information of the image pickup device being caused to have correspondence by the position correspondence control section based on the position information of the image pickup device in the second coordinate system; an associated image generating section configured to generate an image in which the position information related to the branch region where the lumen branches is associated with the image information of the three-dimensional shape of the specific luminal organ by superposedly displaying the position information related to the branch region where the lumen branches on the image information of the three-dimensional shape of the specific luminal organ; a determination section configured to determine whether the position information of the image pickup device in the first coordinate system, which is caused to have correspondence by the position correspondence control section based on the position information of the image pickup device in the second coordinate system, indicates the image pickup device in the first coordinate system is positioned inside or outside the specific luminal organ in the image information of the three-dimensional shape in the first coordinate system; and a control section configured to perform control to display, on a display, a piece of the position information related to the branch region where the lumen branches in a vicinity of the position information of the image pickup device in the specific luminal organ when the determination section determines that the position information of the image pickup device in the first coordinate system is positioned outside of the specific luminal organ, wherein, when the determination section determines that the image pickup device in the first coordinate system is positioned outside the specific luminal organ, the control section corrects the position correspondence information on the basis of a reference position set in the vicinity of the position information of the image pickup device.