Provided is an optical fiber that is suitable for high-density packing and long-haul transmission. An optical fiber according to the present invention includes a core and a cladding. At a wavelength of 1550 nm, an effective area Aeff is 100 µm 2 or less and a chromatic dispersion Disp is 19.0 ps/nm/km or more and 22 ps/nm/km or less, and, when an effective length is denoted by Leff and an attenuation is denoted by ±, a figure of merit FOM represented by an expression "FOM = 5 log{|Disp|·Leff} - 10 log{Leff/Aeff}-100±" is 3.2 dB or more.
