The present invention provides an ultra-low-loss optical fiber comprising: a core part positioned at the center of the optical fiber and having the maximum refractive index in the optical fiber; an inner cladding part disposed outside the core part and having the minimum refractive index in the optical fiber; and an outer cladding part disposed outside the inner cladding part and having a refractive index lower than that of the core part and higher than that of the inner cladding part, wherein the relative refractive index difference (Δ+) between the refractive index of the core part and that of the outer cladding part is 0.0040 to 0.0055 and the relative refractive index difference (Δ-) between the refractive index of the inner cladding part and that of the outer cladding part is -0.0010 to -0.0003.
