PURPOSE: To enable always uniform fixing by forming a tapered part which is gradually reduced in outside diameter toward the rear end to the rear end of a ferrule and inserting a lock ring having the bore smaller than the max. outside diameter of the tapered part onto said tapered part.

CONSTITUTION: A plug 1 has a cylindrical ferrule 2 having an insertion hole 2a into which the core 3a of an optical fiber 3 is inserted and four movable pieces 5 formed with slits 4. Window holes 6 engaging with a connector are formed to a pair of the pieces 5 facing each other. Said plug is formed of a lock ring 10 which fixes an optical fiber 3 by being fitted to a tapered part 9 having a cap 7 fitted freely turnably to the ferrule 2 and slits 8 formed to the end of the ferrule 2 and a protective part which has a recess 11a to fix onto the ring 10 and protects the fiber 3 against disconnection by the bending of said fiber. The tapered part 9 is converged toward the end and the slits 8 are punched axially from the rear end face of the ferrule 2. The tapered part 9 is divided to plural parts by the slits 8 and deflecting pieces 9a are thus formed.