Abstract: 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 ?1 of the core 11 is in the range of 0.3% to 0.38%, the relative refractive index difference ?2 of the first cladding 12 is equal to or smaller than 0%, and the relative refractive index difference ?3 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<r3<0.2r2+16”, and the inner radius r2 of the second cladding 13 is equal to or greater than 8 ?m. The bending loss at a wavelength of 1550 nm and at a radius of curvature of 7.5 mm is smaller than 0.