An optical fiber that has a small bending loss can be securely prevented from being fractured due to accidental bending during installation or other operations. The optical fiber includes a core, a first cladding, a second cladding, and a third cladding. The relative refractive index difference Δ1 of the core is in the range of 0.3% to 0.38%, the relative refractive index difference Δ2 of the first cladding is equal to or smaller than 0%, and the relative refractive index difference Δ3 of the second cladding is in the range of −1.8% to −0.5%. The inner radius r2 and the outer radius r3 of the second cladding satisfy the expression “0.4r2+10.5