The invention relates to a device (1) for shortening an elongated bone, in particular an ulna, comprising a bone plate (5) to be attached to the bone with a hole (6) for a fastening element for fixing the bone plate (5) on the bone and a slot offset longitudinally thereto for a holding element (9) that can be fastened in the bone, so that after the bone has been severed and a piece of bone has been removed in the region between the hole (6) and the slot, bone parts can be guided together by the displacement of the bone plate relative to the holding element (9). In order to create a device (1), which is constructed with a small space requirement and permits a highly accurate guiding together of the bone parts, it is provided according to the invention that a block (12) with an opening (13) for the holding element (9), which opening is elongated in plan view and lies above the slot, is arranged on the bone plate (5) in the region of the slot, wherein a unit (14) is arranged in the opening (13), against which unit the holding element (9) rests in the displacement direction and relative to which a remaining part of the block (12) can be displaced with the aid of a provided adjustment element (15).
