Buckle, particularly for the heel strap of open heel swim fins

A buckle for a fastening strap of an object comprising a sequence of transverse teeth (C1), on one of its sides and is adjustably connected, at least at one of its ends, to the object to be fastened, at one side of the object, by means of the buckle (F), which comprises a strap-guiding member (13), extending on a non-toothed side of the strap (C), transverse thereto, and around which said one of its ends is returned, and a movable strap-locking tooth (22), extending, transverse to the strap (C), on the toothed side thereof, in coincidence with the strap-guiding member (13), and cooperating, together with a counteracting elastic means (24), with the teeth (C1) of the strap (C), in such a way that the strap is only allowed to slide freely, subject to a manual pulling force exerted on a free returned end (C2) of the strap, towards (A) tightening the strap, whereas the strap is prevented from sliding in an opposite direction towards loosening the strap, a hand-operated strap-releasing means (17), for moving the strap-locking tooth (22) away from the strap-guiding member (13), against the action of the associated counteracting elastic means (24), to such an extent as to disengage the strap-locking tooth (22) from the strap teeth (C1) and to allow the strap to slide freely in both senses only as long as the manual action is exerted on the strap-releasing means (17), characterized in that the buckle (F) consists of a base part (F1), attached to the object to be fastened, to one side of the object and comprising the movable strap-locking tooth (22), its respective counteracting elastic means (24) and manual strap-releasing means (17), and of a strap-fastening part (F2), which only comprises strap-guiding means (13) for returning the strap (C), and is joined to the base part (F1), in such a way as to be able to slide in the longitudinal direction of the strap (C) and between a retracted strap-tightening position, wherein it is hooked to the base part (F1) of the buckle, in a manually releasable way, and the strap-guiding member (13) is close to the strap-locking tooth (22), such that said tooth (22) is operatively engaged with the teeth (C1) of the strap (C) and only allows the strap to slide freely towards (A) tightening it, and a forward strap-sliding position, wherein the strap-guiding member (13) is moved away from the strap-locking tooth (22), to such an extent, that said tooth (22) is disengaged from the teeth (C1) of the strap (C) and allows the strap to slide freely in both senses.