Provided is 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. An optical fiber 1 includes a core 11, a first cladding 12, a second cladding 13, and a third cladding 14. The relative refractive index difference Δ1 of the core 11 is in the range of 0.3% to 0.38%, the relative refractive index difference Δ2 of the first cladding 12 is equal to or smaller than 0%, and the relative refractive index difference Δ3 of the second cladding 13 is in the range of −1.8% to −0.5%. The inner radius r2 and the outer radius r3 of the second cladding 13 satisfy the expression “0.4r2+10.5