Abstract: A wire fastening device and a wire extending structure capable of extending a wire with an appropriate tension between support. The wire fastening device includes a case, a cap bolt and a pulling cylinder. The case has a wire engaging portion at one end thereof and a female thread formed on an inner circumference thereof. The cap bolt is inserted into the pulling cylinder with the head abutted with the pulling cylinder and has a male thread formed on an outer circumference thereof. The pulling cylinder is held on the cap bolt rotatably and the case, with one end of the wire engaged therewith, is screwed on the pulling cylinder.