PURPOSE: To enable the accurate disposition of an optical fiber at the center of a plastic plug by closing a die by the optical fiber-base spherical body having a symmetrical shape with respect to the optical fiber axial line thereby forming the plastic plug.

CONSTITUTION: The through-hole 5 of the die 4 for casting fits precisely to the sleeve 3 put on the jacket 1 of an optical fiber cable. The exposed end of an optical fiber 2 is melted by heating to form a spherical body 6 by surface tension. The diameter of the spherical body 6 becomes larger than the diameter of the through- hole 5 and while the spherical body 6 is maintaining the symmetrical shape, it is set. Casting resin 7 is poured into the through-hole 5. The optical fiber 2 is pulled donwward to let the spherical body 6 contact the opening edge of the through-hole 5 and align the centers of the optical fiber 2 and through-hole 5, thence the casting resin 7 is set. Part of the optical fiber 2 including the spherical body 6 and part of the solidified casting resin 1 are removed together with part of the die 4 and the optical fiber cable is pulled from the die 4.