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 (r.sub.1 /r.sub.2) of a first core radius r.sub.1 to a second core radius r.sub.2 and a ratio Y (n.sub.2 /n.sub.1) of a refractive index difference n.sub.2 between the second core and the clad to a refactive index difference n.sub.1 between the first core and the clad are in a range that satisfies the expressions Y.gtoreq.-1.134X+1.0518 (1), and X.ltoreq.-1.4842Y.sup.2 +1.1097Y+0.7097 (2), and provided that Y<0.4 or X.gtoreq.0.76 and that coordinate pairs X=0.8 and Y=0.35 as well as X=0.8 and Y=0.45 are excluded. The plastic optical fiber can have a high transmission power and a wide bandwidth. A cable and an optical transimission device can each include this plastic optical fiber.