A medical apparatus includes a model-image generating section configured to generate a model image obtained by modeling a shape of an inside of a subject, a coordinate calculating section configured to detect a three-dimensional position of a feature point of the inside of the subject, set a polar coordinate on the basis of a position of the feature point, and calculate an arbitrary three-dimensional position of the inside of the subject according to the polar coordinate, and an image generating section configured to show the arbitrary three-dimensional position of the inside of the subject on the model image on the basis of one angle component among components of the polar coordinate calculated by the coordinate calculating section and a value obtained by correcting the one angle component according to another angle component.