The present invention provides a string fastening device which allows a string to be threaded thereinto easily, facilitates tightening of the string and enables the string to be gripped softly and captured accurately. The string fastening device can be used with its main body only and the main body includes a penetrating hole comprised of a sliding passage portion which allows the string to be moved freely and a braking passage portion for restricting movement of the string such that they adjoin. A narrow portion is provided at a joint portion so as to prevent the string from changing its path freely. The main body is made of thermoplastic elastomer to have elasticity. A rear face of the main body is formed to be a flat face. An attachment groove or sewing holes for sewing operation are provided around the penetrating hole. The main body is placed on a string threading hole in the cloth to be attached by a sewing thread to be fixed securely.