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, and is compliant with the G. 652 standard. 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