A ferrule includes a body having a cylindrical shape. Elastic gripping portions are arranged on the body so as to bulge radially inwardly. The elastic gripping portions can have elastic contact with the outer circumferential surface of a glass tube. The body of the ferrule is mounted to the glass tube via the elastic gripping portions capable of elastic and radial deformation. Thereby, the variations in outer diameter of the glass tube can be absorbed, and therefore the dimensional control with high precision is not required for glass tubes at the time of manufacturing. This leads to cost reduction.