An optical fiber includes: a core portion; and a cladding portion configured to surround an outer periphery of the core portion, the cladding portion having a refractive index lower than a maximum refractive index of the core portion, and containing a dopant to reduce the refractive index. The core portion includes a center core in which an average refractive index is maximized in the optical fiber, a relative refractive-index difference of the average refractive index of the center core with respect to an average refractive index of the cladding portion is 0.20% to 0.50%, an outer diameter of the cladding portion is 70 μm to 120 μm, a cable cutoff wavelength is equal to or smaller than 1530 nm, and a transmission loss at a wavelength of 1550 nm is equal to or less than 0.18 dB/km.