The invention relates to an extension device for a bone anchor, in particular for a bone screw, in particular for a pedicle screw, in particular in minimally invasive spinal surgery, the device having an axial longitudinal direction, a peripheral direction concentric therewith, and a radial direction, wherein the extension device extends in the axial longitudinal direction and engages, by means of a radial projection, an engageable region on the head of the bone anchor in the longitudinal direction and also in the radial direction and can be supported against the head of the bone anchor in the opposite direction so that the extension device can thereby be fixed to the head of the bone anchor so as to be detachable, yet also rigid in the longitudinal direction and non-rotatable.
