An optical fiber comprising: a core at a center thereof; a first cladding adjacent to the core to cover a circumference of the core; and a second cladding adjacent to the first cladding to cover a circumference of the first cladding, characterized in that 0.35%≤(Δ 1 -Δ 2)≤0.65%, 0.30%≤ Δ 1 ≤0.55%, -0.20%≤Δ 2 ≤-0.05%, 0.22≤a / b≤0.34, and 4≤b √|Δ 2 |≤10 hold, and loss increase resulting when the optical fiber is wound on a mandrel having a diameter of 20mm is 0.5dB/turn or smaller at a wavelength of 1550nm, where Δ 1 (%) is a specific refractive index difference of a maximum of a refractive index distribution of the core with reference to a refractive index of the second cladding, Δ 2 (%) is a specific refractive index difference of a minimum of a refractive index distribution of the first cladding with reference to the refractive index of the second cladding, a(μm) is a radius of a boundary between the core and the first cladding with respect to a center of the core, and b(μm) is a radius of a boundary between the first cladding and the second cladding with respect to the center of the core, whereby an expression of Δ(r)=(Δ 1 *-Δ a)(1-r/a) α +Δ a holds (in the optical fiber), where Δ(r) (%) represents a specific refractive index difference for a refractive index at a radius r(μm) from the center with reference to a refractive index of the second cladding, Δ1* represents a fitting coefficient, Δ a represents a specific refractive index difference (%) of a refractive index of r=a with respect to the refractive index of the second cladding, and α is a multiplier in a range of 2 and 5, inclusive. a core at a center thereof; a first cladding adjacent to the core to cover a circumference of the core; and a second cladding adjacent to the first cladding to cover a circumference of the first cladding, characterized in that 0.35%≤(Δ 1 -Δ 2)≤0.65%, 0.30%≤ Δ 1 ≤0.55%, -0.20%≤Δ 2 ≤-0.05%, 0.22≤a / b≤0.34, and 4≤b √|Δ 2 |≤10 hold, and loss increase resulting when the optical fiber is wound on a mandrel having a diameter of 20mm is 0.5dB/turn or smaller at a wavelength of 1550nm, where Δ 1 (%) is a specific refractive index difference of a maximum of a refractive index distribution of the core with reference to a refractive index of the second cladding, Δ 2 (%) is a specific refractive index difference of a minimum of a refractive index distribution of the first cladding with reference to the refractive index of the second cladding, a(μm) is a radius of a boundary between the core and the first cladding with respect to a center of the core, and b(μm) is a radius of a boundary between the first cladding and the second cladding with respect to the center of the core, whereby an expression of Δ(r)=(Δ 1 *-Δ a)(1-r/a) α +Δ a holds (in the optical fiber), where Δ(r) (%) represents a specific refractive index difference for a refractive index at a radius r(μm) from the center with reference to a refractive index of the second cladding, Δ1* represents a fitting coefficient, Δ a represents a specific refractive index difference (%) of a refractive index of r=a with respect to the refractive index of the second cladding, and α is a multiplier in a range of 2 and 5, inclusive.