A plastic optical fiber (POF) is provided which includes a core layer constructed such that a portion at least a certain distance away from a core center, as seen in cross section, contains a plurality of regions of polymer materials having different refractive indices, wherein the average refractive index per circumference continuously decreases with the increase in distance from the core center and wherein the refractive indices are discontinuously distributed on each whole circumference at each distance from the core center.