An extension device for a bone anchor is provided, wherein the bone anchor comprisesan anchoring section (100) for anchoring in a bone anda receiving part (200, 200) connected to the anchoring section, the receiving part comprising a central axis (C) and a channel (204) for receiving a rod, wherein sidewalls of the channel form two free legs (205a, 205b),the extension device comprisinga first sleeve (1, 1) with a first sleeve axis (c) that is coaxial with the central axis (C), wherein the first sleeve (1, 1) is configured to be detachably coupled to the receiving part (200, 200) anda second sleeve (2, 2, 2", 2") with a second sleeve axis, wherein the second sleeve is positioned within the first sleeve (1, 1),wherein the first sleeve (1, 1) is configured to be coupled to the receiving part (200, 200) such as to inhibit a translational movement of the first sleeve relative to the receiving part along the central axis andwherein the second sleeve (2, 2, 2", 2") is configured to be coupled to the receiving part so as to hinder or inhibit a rotational movement of the second sleeve (2, 2, 2", 2") relative to the receiving part (200, 200) and wherein the second sleeve (2, 2, 2", 2") is connected to the first sleeve (1, 1) through a coupling member (3) that is configured to advance in the first sleeve (1, 1) together with the second sleeve (2, 2, 2", 2") in an axial direction.
