The invention relates to a closure buckle for closing a ski boot, which buckle comprises a main lever (2) hinged to a base (1) and having a longitudinal notch (20) in which a tie rod (3) is mounted that is connected to a fastener hook and that has an end carrying an auxiliary lever (4), said buckle being characterized in that the main lever (2) is provided with a protective plate (21) laterally defining a housing (24) for receiving the auxiliary lever (4), and the invention also relates to a downhill ski boot equipped with such buckles.