A plastic optical fiber (11) has a core (12) and a clad (13). The clad (13) is composed of an outer clad (14) and an inner clad (15). The refractive index in the core (12) gradually increases as the distance from the center thereof decreases. The refractive index in the inner clad (15) is equal to the minimum value of the refractive index in the core (12), and the refractive index in the outer clad (14) is smaller than that in the inner clad (15). For the purpose of decreasing the transmission loss between the plastic optical fiber (11) and a light emission device or the light receiving device, the diameter d1 of the core (12) and the outer diameter d2 of the inner clad (15) satisfy the following conditions; 100(.mu.m).ltoreq.d1.ltoreq.700(.mu.m) 200(.mu.m).ltoreq.d1.ltoreq.1000(.mu.m) d1