A fastening device is manufactured of a strip of sheet material and comprises a flexible tongue capable of being looped around an element to be fastened and a holding part. The tongue has a hook-shaped free end and an end opposite to the hook-shaped free end, and the holding part projects at an angle from the opposite tongue end and is bendable towards the tongue along a folding line adjacent the opposite tongue end. The holding part is integral with the tongue and has a hook-up edge spaced from the tongue end and arranged to be engaged by the hook-shaped free end when the flexible tongue is looped around the element to be fastened, and it is operable as a lever upon bending the holding part towards the tongue along the folding line for tightening and closing the fastening device when the tongue is looped around the element to be fastened and the hook-up edge of the holding part has been engaged by the hook-shaped free end of the tongue.