An optical fiber includes a clad portion and a core portion surrounded by the clad portion. Here, the clad portion is formed by substantially pure quartz, and the core portion has a higher refractive index than the clad portion. The optical fiber is characterized in that a relative refractive index difference of a center of the core portion falls within a range from 0.15% to 0.30%, and a maximum relative refractive index difference of the core portion falls within a range from 0.4% to 0.6%. Here, it is preferable that an average relative refractive index difference of the core portion falls within a range from 0.30% to 0.40%. Here, a mode field diameter of the optical fiber falls within a range from 8.6 .mu.m to 9.5 .mu.m at 1310 nm, and a cable cutoff wavelength of the optical fiber is equal to or lower than 1260 nm. Note that, after the optical fiber is subjected to hydrogen aging, a loss of the optical fiber is preferably equal to or lower than 0.4 dB/km at 1383 nm.