<p>The present invention provides a plastic optical fiber comprising a cladding, a first core disposed inside the cladding, and a second core disposed inside the first core; wherein a ratio X of a diameter (D2) of the second core to a diameter (D1) of the first core (=D2/D1) and a ratio Y of a difference (n1 - n0) between a refractive index (n1) of the first core and a refractive index (n0) of the cladding to a difference (n2 - n0) between a refractive index of the second core (n2) and the refractive index (n0) of the cladding (=(n1 - n0)/(n2 - n0)) satisfy the following relational expressions: 0.3 ≤ X ≤ 0.95, 0.4 ≤ Y ≤ 0.95, 1 ≤ X + Y ≤ 1.9. <IMAGE></p>
