When plug is fitted to adapter, optical fiber is inserted to an alignment hole for the optical fiber. By providing a taper at the entrance thereof, even if the tip of the optical fiber is deviated from the central position, the deviation is absorbed to make it possible for the optical fiber to be inserted smoothly. When two plugs is fitted to adapter, the tip of one optical fiber is confronted with the other one in the alignment hole, and excessive length L.sub.1 -L.sub.2 is pushed into the two optical fiber-bending spaces with a suitable distribution, thus the optical fiber buckles therein. The end face of the optical fiber is subjected to flat surface-polishing, removal of ripples after being cleaved, or chamfer grinding after being cleaved. The buckling portion of the optical fiber is coated with carbon or the like.