This invention relates to an optical fiber and others having a structure for efficiently generating SC light while realizing high nonlinearity over a wide band. The optical fiber has at least a center core region, and an outside cladding region having a refractive index lower than that of the center core region and provided on an outer periphery of the center core region. The optical fiber has, as characteristics to light of a wavelength .lamda.1 in a wavelength range of 1520 nm to 1620 nm, a chromatic dispersion of -2 ps/nm/km to +2 ps/nm/km, a dispersion slope of -0.009 ps/nm.sup.2/km to +0.009 ps/nm.sup.2/km, and a fourth-order dispersion of -1.8.times.10.sup.-4 ps/nm.sup.3/km to +1.8.times.10.sup.-4 ps/nm.sup.3/km.