Provided is an optical fiber that is in particular suitable for Raman amplification. An effective area at a wavelength of 1570 nm is in a range of 35 &mgr;m2 to 45 &mgr;m2, an absolute value of a dispersion slope at the wavelength is equal to or less than 0.04 ps/nm2/km, and a dispersion value at the wavelength is in a range of 5 ps/nm/km to 10 ps/nm/km. It is desirable that a refractive index profile contains at least one annular region between a center core and a cladding region.