An optical fiber includes a first core with a first refractive index located in a central portion of the optical fiber; a second core with a second refractive index located in an outer periphery of the first core; a third core with a third refractive index located in an outer periphery of the second core; and a cladding with a fourth refractive index located in an outer periphery of the third core, where among the refractive indices, the first one>the third one>the fourth one>the second one. The absolute value of dispersion at the wavelength of 1550 nm is not more than 20 ps/nm/km. The effective area at the wavelength is not more than 15 .mu.m.sup.2. The nonlinear constant n.sub.2/Aeff at the wavelength of 1550 nm is equal to or more than 25.times.10.sup.-10/W.