An optical fiber that transmits a signal light in a fundamental propagation mode has a cutoff wavelength longer than a wavelength of the signal light, a wavelength dispersion of the fundamental propagation mode of -5 ps/nm/km to -1 ps/nm/km at a wavelength of 1550 nanometers, an effective core area of the fundamental propagation mode larger than 45 .mu.m.sup.2 at the wavelength of 1550 nanometers, and a dispersion slope of the fundamental propagation mode smaller than 0.03 ps/nm.sup.2/km at the wavelength of 1550 nanometers.