A plastic optical fiber includes having a first core, a second core having a different refractive index from the first core, disposed concentrically on the outside of the first core, and a clad disposed concentrically on the outside of the second core, wherein a ratio X (r1/r2) of a first core radius r1 to a second core radius r2 and a ratio Y (n2/n1) of a refractive index difference n2 between the second core and the clad to a refactive index difference n1 between the first core and the clad are in a range that satisfies the expressions Y≧−1.134X+1.0518 (1), and X≦−1.4842Y2+1.1097Y+0.7097 (2), and provided that Y