An intervertebral nucleus prosthesis is proposed, which is characterized in that it consists of at least one, in particular, spherical body movable in two directions of a plane and made of a rigid, non-oxidizing, biocompatible material with a diameter adapted to the biological nucleus, the spherical body being mounted non-displaceably but freely rotatably about its center in a cage and protruding at both opposite sides in the form of a spherical cap from the cage. Also proposed is a method for implanting such a prosthesis.