PURPOSE: To realize the uniformity in holding plug ferrule as well as to reduce the cost of sleeve, by providing plural through holes to the tick wall part of a cylindrical sleeve having a through hole whose inner diameter is smaller than the outer diameter of a plug ferrule and all of which are parallel to the through hole on the coaxial circumference with the through hole.

CONSTITUTION: A center hole 7 of the sleeve is enlarged by the insertion of a plug ferrule 4. In this case, as the thickness A from a curcular arcuate through hole 6 to the center hole 7 is smaller than the thickness B where the through hole 6 is not located, the through hole 6 is easier to extend towards outer side resulting in easier insertion of the plug ferrule 4. Moreover, the resiliency C against the elastic deformation is stronger at the part where no through hole 6 is located, the sleeve forms three point support structure with respect to the ferrule 4, resulting in uniform and stable holding of ferrule as well as price down due to the improvement in manufacturing yield.