Then, filler such as quartz powder is filled in the gaps created between the outermost layer of the capillary bundle and the inner walls of the support pipe 1 to prevent dislocation of the capillaries 2 Thus, as explained above, the preform 4, which is made of numerous capillaries 2, 2, . . . disposed in a densely packed state forming a capillary bundle, the core rod 3 arranged at the central axis