Abstract: The invention concerns a device for lengthening a person's own hair using artificial and/or genuine hair and for the permanent fastening of artificial and/or genuine hair. The invention is characterized in that a tube (12), which can be shrunk when energy is applied, is disposed above a thickened portion, secured in position, in particular a knot in a strand of a person's own hair (16) and above an extension piece of artificial and/or genuine hair. An energy-application device, in particular in the form of heating irons (20), transfers thermal energy in a deliberate manner to the shrinkable tube (12). The method in which the device is used is characterized by the following steps: knotting a strand of the person's own hair, sliding onto the knot in the hair strand a fastening element which can shrink when energy is applied; sliding an extension strand of artificial and/or genuine hair into the tube; and shrinking the shrinkable fastening element by the application of energy.