The present invention relates to an optical fiber having a large positive dispersion in a wavelength band of 1.55 &mgr;m in order to compensate for a negative dispersion inherent in an NZ-DSF in the wavelength band of 1.55 &mgr;m. This optical fiber comprises a depressed cladding structure constituted by a core region; an inner cladding, disposed at the outer periphery of the core region, having a lower refractive index; and an outer cladding having a higher refractive index. In this optical fiber, the relative refractive index difference of the core region with respect to the outer cladding is at least 0.30% but not greater than 0.50%, and the relative refractive index difference of the inner cladding with respect to the outer cladding is at least −0.50% but not greater than −0.02%. Also, the optical fiber has a dispersion greater than 18 ps/nm/km at a wavelength of 1.55 &mgr;m, and an effective cross-sectional area Aeff of at least 70 &mgr;m2 at the wavelength of 1.55 &mgr;m.