An optical fiber comprising: a core comprising a positive maximum refractive index difference Δn1 in the optical fiber; an inner layer comprising a refractive index difference Δn2 that is smaller than the positive maximum refractive index difference of the core and decreases in a direction away from the core, the inner layer being positioned outside the core; and a trench layer comprising a negative inner-circumference refractive index difference Δn3 that is smaller than the refractive index difference of the inner layer and a negative outer-circumference refractive index difference Δn4 that is a minimum refractive index difference in the optical fiber, the trench layer being positioned outside the inner layer, wherein a refractive index difference among layers of the optical fiber is defined as a difference between a refractive index of each layer and a refractive index of an outer layer positioned outside the trench layer, characterized in that a refractive index difference of the trench layer gradually decreases from an inner surface to an outer surface of the trench layer, a ratio (Δn3/Δn4) of the negative inner-circumference refractive index difference Δn3 to the negative outer-circumference refractive index difference Δn4 is larger than 0.6 and smaller than 1, a ratio (a+b)/b of a radius 'a' of the core and a thickness 'b' of the inner layer is 2.8 or less, and the optical fiber comprises a cutoff wavelength of 1260nm or less, a bending loss α1 of 1.0dB or less in bending by a radius of 10mm or less at a wavelength of 1550nm, and a ratio (α2/α1) of a bending loss α2 at 1625nm to the bending loss α1 at 1550nm being less than 2.7. a core comprising a positive maximum refractive index difference Δn1 in the optical fiber; an inner layer comprising a refractive index difference Δn2 that is smaller than the positive maximum refractive index difference of the core and decreases in a direction away from the core, the inner layer being positioned outside the core; and a trench layer comprising a negative inner-circumference refractive index difference Δn3 that is smaller than the refractive index difference of the inner layer and a negative outer-circumference refractive index difference Δn4 that is a minimum refractive index difference in the optical fiber, the trench layer being positioned outside the inner layer, wherein a refractive index difference among layers of the optical fiber is defined as a difference between a refractive index of each layer and a refractive index of an outer layer positioned outside the trench layer, characterized in that a refractive index difference of the trench layer gradually decreases from an inner surface to an outer surface of the trench layer, a ratio (Δn3/Δn4) of the negative inner-circumference refractive index difference Δn3 to the negative outer-circumference refractive index difference Δn4 is larger than 0.6 and smaller than 1, a ratio (a+b)/b of a radius 'a' of the core and a thickness 'b' of the inner layer is 2.8 or less, and the optical fiber comprises a cutoff wavelength of 1260nm or less, a bending loss α1 of 1.0dB or less in bending by a radius of 10mm or less at a wavelength of 1550nm, and a ratio (α2/α1) of a bending loss α2 at 1625nm to the bending loss α1 at 1550nm being less than 2.7.