The present invention relates to an optical fiber which has a structure for further increasing an FOM (=|dispersion|/loss) and which can be applied to a dispersion compensation module. The optical fiber is mainly composed of silica glass and has a core region including a center of an optical axis, a depressed region surrounding the core region, a ring region surrounding the depressed region, and a cladding region surrounding the ring region and doped with F. As compared with the refractive index of pure silica glass, a relative refractive index difference of the core region is 3% or more but 4% or less, a relative refractive index difference of the depressed region is -1% or more but -0.5% or less, a relative refractive index difference of the ring region is 0.01% or more but 0.24% or less, and a relative refractive index difference of the cladding region is -0.3% or more but -0.1% or less. The FOM at the wavelength of 1550 nm is 250 ps/nm/dB or more.