The present invention relates to an optical fiber which can improve OSNR in an optical transmission system in which Raman amplification and an EDFA are combined. With respect to the optical fiber, a predetermined conditional formula is satisfied by an effective area Aeff1450 [μm2] at a wavelength of 1450 nm, a transmission loss α1450 [/km] at a wavelength of 1450 nm, and a transmission loss α1550_dB [dB/km] at a wavelength of 1550 nm. Further, with respect to the optical fiber, another predetermined conditional formula is satisfied by an effective area Aeff1550 [μm2] at a wavelength of 1550 nm, and a transmission loss α1550 [/km] at a wavelength of 1550 nm.