An optical fiber and an optical fiber device, which can suppress generation of the stimulated Brillouin scattering while maintaining large nonlinearity. The fiber includes a central core which is made of SiO.sub.2 as a main component and contains Al.sub.2O.sub.3 at concentration of 15 wt % or more. The fiber is allowed to propagate only a fundamental mode and has an absolute value of chromatic dispersion of 5 ps/nm/km or less at a predetermined wavelength. The device includes the fiber having the above composition. The fiber guides first light and second light having different wavelengths to propagate therethrough such that the first light and the second light interact with each other by a nonlinear optical phenomenon during the propagation, thus causing the fiber to output the first light or the second light modulated by the nonlinear optical phenomenon or third light newly produced by the nonlinear optical phenomenon and having a different wavelength.