The invention relates to an automatic heel unit (1) for a ski binding, comprising a base (7) for fitting the automatic heel unit (1) on a ski and a heel downholder (3) for holding down a ski boot in a heel region of the ski boot. The heel downholder (3) is mounted so as to be movable in relation to the base (7). The automatic heel unit (1) has a holding configuration in which the heel downholder (3) is located in a holding position and may interact with the heel region of the ski boot that is held in the ski binding in such a manner that the heel region of the ski boot is held down in a lowered position. Furthermore, the automatic heel unit (1) has a step-in configuration in which the heel downholder (3) is located in a step-in position and the heel region of the ski boot is released by the heel downholder (3). The automatic heel unit (1) comprises a heel support structure (6), which is configured separately from the heel downholder (3), for supporting, in a direction that is horizontally transverse to the ski, the heel region of the ski boot that is held in the ski binding in the holding configuration of the automatic heel unit (1).

Ski boot comprising a rigid shell that is shaped so as to accommodate the foot of the user, and a rigid cuff that is shaped to as to surround the ankle of the user, and is pivotally jointed to the shell so as to be able to pivot about a rotation axis substantially perpendicular to the midplane of the boot; the shell comprising a substantially basin-shaped, rigid casing which is shaped so as to be able to accommodate therein the foot of the user, and a transversal stiffening insert which is embedded inside the bottom wall of the casing at the tarsophalangeal region of the sole of the foot, and is structured so as to emerge/surface on the outside of the rigid casing on opposite sides of the midplane of the boot, at the two lateral sides of the tip of the shell; the transversal stiffening insert being provided with two first conjugated coupling ends that surface on the outside of the rigid casing along the two lateral sides of the tip of the shell, on opposite sides of the midplane of the boot, and are structured so as to be engaged and clamped in an axially rotating manner about a transversal reference axis by the toe piece of the ski binding device, and of two second conjugated coupling ends that surface on the outside of the rigid casing along the two lateral sides of the tip of the shell, on opposite sides of the midplane of the boot, spaced beside said first conjugated coupling ends.