A wide band optical fiber having a suitable wide band for communication, hardly breaking and cracking even when a press-fit type connector is connected by a sufficient pressing force having a small loss due to the press-fit connection and a high alignment accuracy, capable of minimizing a connecting loss and being connected easily to an ordinary quartz GI type optical fiber of 125 mu m diameter, and having a high interchangeability with an ordinary quartz GI type optical fiber. The wide band optical fiber is a refractive index distribution type optical fiber comprising a core of quartz glass having a refractive index distribution, a clad of quartz glass provided closely on the core, and a polymer covering layer provided closely on the clad, and characterized in that the polymer covering layer is made of an ultraviolet ray-cured hard polymer having a Shore hardness of above D55 and a thickness of above 5 mu m. Primary and secondary covering layers are provided closely on this optical fiber to obtain an optical fiber core wire. A polymer sheath layer is provided on the optical fiber core wire to obtain an optical fiber cord.