In the capillary tube 22, the cross section of the outer surface is substantially square, and the cross section of the insertion hole 23 is substantially square. Sides La, Lb of the outer surface have high dimensional accuracy and the insertion hole 23 is made to allow two optical fibers 5, 6 to be inserted while adjoining each other and arranged in order therein. The phases of the outer surface and the insertion hole 23 are relatively offset with each other, so that angles &thgr;a, &thgr;b which the sides 23a, 23b of the insertion hole 23 form with flat surfaces 22a, 22b of the outer surface are acute angles of substantially 45°±0.5°. Consequently, the flat surface 22a becomes substantially parallel to the center line M that connects the centers of the cores 5a, 6a of the two optical fibers 5, 6 inserted in the insertion hole 23, and the flat surface 22b becomes substantially perpendicular to the center line M.