An optical fiber mainly comprised of silica-based glass, comprising: - a core extending along an optical axis and including an alkali metal element; and - a cladding surrounding the core and having a refractive index lower than that of the core, characterized in that the mean concentration of the alkali metal element in the core or the shape of the concentration distribution of the alkali metal element in the core is adjusted such that the weighted value (I D2 /I ω3) weight of the distribution of the ratio I D2 / I ω3 along the radial direction of the optical fiber is ≤ 0.48, the weighted value being expressed as I D 2 / I ω 3 weight = ∫ 0 2 π ∫ 0 10 E r 2 ⋅ I D 2 I π 3 rdrdφ ∫ 0 2 π ∫ 0 10 E r 2 rdrdφ wherein I ω3 is the intensity of Raman scattering light ω3 by a Si-O stretching vibration in a Raman scattering spectrum, I D2 is the intensity of Raman scattering light D2 by a silica three-membered ring structure, r is the distance from the optical axis, and E ( r) is the field intensity at the position of the distance r from the optical axis, and being obtained by weighting the distribution of the ratio I D2 / I ω3 with a distribution of field intensity along a radial direction of guided light at a wavelength of 1550 nm, calculated from a refractive index profile along the radial direction of the optical fiber, in a region including the optical axis as a center and having a diameter of 20 μm in a cross section of the optical fiber orthogonal to the optical axis, and the shape of the concentration distribution includes a first concentration peak located at the core center including the optical axis and a second concentration peak separated by a predetermined distance from the first concentration peak. - a core extending along an optical axis and including an alkali metal element; and - a cladding surrounding the core and having a refractive index lower than that of the core, characterized in that the mean concentration of the alkali metal element in the core or the shape of the concentration distribution of the alkali metal element in the core is adjusted such that the weighted value (I D2 /I ω3) weight of the distribution of the ratio I D2 / I ω3 along the radial direction of the optical fiber is ≤ 0.48, the weighted value being expressed as I D 2 / I ω 3 weight = ∫ 0 2 π ∫ 0 10 E r 2 ⋅ I D 2 I π 3 rdrdφ ∫ 0 2 π ∫ 0 10 E r 2 rdrdφ wherein I ω3 is the intensity of Raman scattering light ω3 by a Si-O stretching vibration in a Raman scattering spectrum, I D2 is the intensity of Raman scattering light D2 by a silica three-membered ring structure, r is the distance from the optical axis, and E ( r) is the field intensity at the position of the distance r from the optical axis, I ω3 is the intensity of Raman scattering light ω3 by a Si-O stretching vibration in a Raman scattering spectrum, I D2 is the intensity of Raman scattering light D2 by a silica three-membered ring structure, r is the distance from the optical axis, and E ( r) is the field intensity at the position of the distance r from the optical axis, and being obtained by weighting the distribution of the ratio I D2 / I ω3 with a distribution of field intensity along a radial direction of guided light at a wavelength of 1550 nm, calculated from a refractive index profile along the radial direction of the optical fiber, in a region including the optical axis as a center and having a diameter of 20 μm in a cross section of the optical fiber orthogonal to the optical axis, and the shape of the concentration distribution includes a first concentration peak located at the core center including the optical axis and a second concentration peak separated by a predetermined distance from the first concentration peak.