Abstract: A conversion adapter to be used for connecting opposed optical connector ferrules having different diameters is formed of an amorphous alloy possessing at least a glass transition region, preferably a glass transition region of not less than 30K in temperature width. Particularly, the amorphous alloy of M1-M2 system or M1-M2-La system (M1: Zr and/or Hf, M2: Ni, Cu, Fe, Co, Mn, Nb, Ti, V, Cr, Zn, Al, and/or Ga, La: rare earth element) possesses a wide range of &Dgr;Tx and thus can be advantageously used as a material for the conversion adapter. Such a conversion adapter can be manufactured with high mass-productivity by a mold casting method or molding method.