An optical fiber comprising: a core portion made of silica based glass; a cladding portion made of silica based glass and configured to cover an outer periphery of the core portion, the cladding portion having a refractive index that is lower than a maximum refractive index of the core portion; and a coating portion configured to cover an outer periphery of the cladding portion, the coating portion including a primary coating layer located on the cladding portion side, and a secondary coating layer located on an outer peripheral side of the primary coating layer, wherein an outer diameter of the cladding portion is larger than or equal to 75 μm and less than 100 μm, a thickness of the primary coating layer is larger than or equal to 15 μm and smaller than or equal to 30 μm, a mode field diameter at a wavelength of 1310 nm is larger than or equal to 8.6 μm and smaller than or equal to 9.2 μm, an effective cutoff wavelength is smaller than or equal to 1260 μm, a bending loss at a wavelength of 1550 nm when bending is made at a diameter of 60 mm is smaller than or equal to 0.1 dB/100 turn, the core portion includes a central core portion and a depressed layer that is formed an outer periphery of the central core portion, the optical fiber has a W-type refractive index profile, a relative refractive-index difference Δ1 of the core portion with respect to the cladding portion is larger than or equal to 0.33% and smaller than or equal to 0.40%, when a relative refractive-index difference of the depressed layer with respect to the cladding portion is Δ2, Δ2 is larger than or equal to -0.20% and less than 0%, and when a core diameter of the central core portion is 2a, and an outer diameter of the depressed layer is 2b, b/a is smaller than or equal to 5. a core portion made of silica based glass; a cladding portion made of silica based glass and configured to cover an outer periphery of the core portion, the cladding portion having a refractive index that is lower than a maximum refractive index of the core portion; and a coating portion configured to cover an outer periphery of the cladding portion, the coating portion including a primary coating layer located on the cladding portion side, and a secondary coating layer located on an outer peripheral side of the primary coating layer, wherein a primary coating layer located on the cladding portion side, and a secondary coating layer located on an outer peripheral side of the primary coating layer, wherein an outer diameter of the cladding portion is larger than or equal to 75 μm and less than 100 μm, a thickness of the primary coating layer is larger than or equal to 15 μm and smaller than or equal to 30 μm, a mode field diameter at a wavelength of 1310 nm is larger than or equal to 8.6 μm and smaller than or equal to 9.2 μm, an effective cutoff wavelength is smaller than or equal to 1260 μm, a bending loss at a wavelength of 1550 nm when bending is made at a diameter of 60 mm is smaller than or equal to 0.1 dB/100 turn, the core portion includes a central core portion and a depressed layer that is formed an outer periphery of the central core portion, the optical fiber has a W-type refractive index profile, a relative refractive-index difference Δ1 of the core portion with respect to the cladding portion is larger than or equal to 0.33% and smaller than or equal to 0.40%, when a relative refractive-index difference of the depressed layer with respect to the cladding portion is Δ2, Δ2 is larger than or equal to -0.20% and less than 0%, and when a core diameter of the central core portion is 2a, and an outer diameter of the depressed layer is 2b, b/a is smaller than or equal to 5. <EOS>