An attachment device for tightening an adjustable resistant strap, comprising a buckle composed of a pair of superposed rings of different dimensions. The buckle is associated with a positioning shim housed around a closed first turn arranged opposite the blocking slot of the strap. The positioning shim is configured to separate the top strand and the bottom strand of said turn from one another. This results in a reduction of the friction effect when relative movement takes place between the two rings. The shim is equipped for this purpose with two guide apertures through which the bottom strand and the top strand of the closed turn attached to the buckle respectively pass.