The invention relates to an optical fiber employable in an optical communication system using Raman amplification and adapted to improve OSNR and suppress bending loss at the same time, and the like. The optical fiber is a silica-based optical fiber having a depressed refractive index profile constituted by at least a core, an inner cladding having a low refractive index, and an outer cladding, an effective area Aeff of 110 μm2 or more at the wavelength of 1550 nm, and a fiber cutoff wavelength λc of 1.3 μm or more but 1.53 μm or less. The depressed refractive index profile is designed such that the ratio Ra(=2b/2a) of the diameter of the inner cladding to the diameter of the core is 2.5 or more but 3.5 or less and that the relative refractive index difference Δ− of the inner cladding with respect to the outer cladding is at least the relative refractive index difference Δ−min where the bending loss at the wavelength for use is minimized but not exceeding (Δ−min+0.06) %.