Ski boot comprised of a rigid shell at least partially surmounted by an upper (4) and at least one padding element (9) inside the boot intended to block the skier's foot. The boot comprises a blocking device (12) having a block (16, 16') extending from each side of the zone of the Achilles tendon into the hollow of the malleoli located above the protuberance of the calcaneum, and the block is attached to a block holder (14) provided with a height adjustment device (17, 17'), arranged along the median vertical axis of the back of the upper.