Provided is an optical fiber including: a first core at a center thereof; a second core adjacent to the first core to cover a circumference of the first core; a third core adjacent to the second core to cover a circumference of the second core; and a cladding adjacent to the third core to cover a circumference of the third core, where conditions of 0.28%≦Δ1≦0.4%, −0.05%≦Δ2≦0.05%, −1.0%≦Δ3≦−0.5%, 3.8 μm≦a≦4.5 μm, 12 μm≦b≦21 μm, and 1.5 μm≦c-b≦10 μm hold, and loss increase resulting when the optical fiber is wound on a mandrel having a diameter of 20 mm is 0.1 dB/turn or smaller at a wavelength of 1625 nm, where Δ1 is a specific refractive index difference of the first core from the cladding refractive index, Δ2 is a specific refractive index difference of the second core from the cladding refractive index, Δ3 is a specific refractive index difference of the third core from the cladding refractive index, “a” is a radius of a boundary between the first core and the second core with respect to a center of the first core, “b” is a radius of a boundary between the second core and the third core with respect to the center of the first core, and “c” is a radius of a boundary between the third core and the cladding with respect to the center of the first core.