A plastic ski boot, comprising a molded shell formed of one solid stock with the sole, an upper insole rigidly situated at the bottom of the shell and above the sole, flexible tightening means taking support on the different faces of the shell, intended to firmly tighten the shell to the foot of a skier, wherein at the level of the heel, at the interior of the space formed between the upper insole and the sole, a rigid member connected to the end of the tightening means which, when it is subjected to a pressure exercised on the rear of the boot by the boltlocking of the rear abutment of the connection of the binding to the boot, exerts on said tightening means a force assuring holding of the foot in the interior of the boot.