a preform having a core and a cladding portion is prepared for a refractive index - distributed type plastic optical fiber . the fiber obtained has a large difference in refractive index between the central and peripheral zone of the core portion . this suppresses light propagation modes in the cladding portion , a cause of deteriorated transmission characteristics , without raising the concentration of non - polymerizable compounds contained in the core portion . to manufacture the preform , a cylindrical vessel is prepared from a polymer , which serves as cladding portion . the inner surface of the preform is then provided with a polymer - containing layer having a refractive index lower than that of the polymer used for the core portion , so as to leave a cavity inside this vessel . this cavity is then filled with a monomer , which is subsequently polymerized while dissolving the lower - refractive index , polymer - containing layer , so as to form the core portion .