To provide a conversion sleeve that enables ferrules that are inserted into the conversion sleeve and have different diameters to be continuously subjected to PC contact at a fixed position each other, reduces axis deviation (an offset) in optical fibers each other as much as possible, and hence can stably connect the ferrules having the different diameter each other with a low contact loss continuously without generating any variations.

In the conversion sleeve 1 comprising a large diameter section 2 of the sleeve, a small diameter section 3 of the sleeve, and a taper diameter section 4 for connecting them, a ferrule insertion stop section 6 is provided on the inner-periphery surface of the taper diameter section and/or on the inner- periphery surface of the large and small diameter sections of sleeve adjacent to that of the taper diameter section.