Headrest device for a motor vehicle seat

The device has a head-rest including reinforcement (3) with a tubular metal rod (4) that assembles the reinforcement on a backrest (5) of a seat. A sheath (8) slidingly receives the rod, and a drawer (9) is mounted at sheath top and equipped with an engagement edge (10) of a locking notch (6) of the rod. A spring (11) actuates the drawer toward a locking configuration. A reception structure (12) receives the sheath by fitting. A space (13) is formed between top of the structure and bottom of the edge. The rod has a weakened fold area (7) in the space when the edge is engaged with the notch.