An optical fiber comprising: a core ; and a cladding surrounding the core , wherein the core includes an inner core part including a center axis (C) of the core and having a refractive index that is constant in a radial direction, and an outer core part surrounding the inner core part , the cladding includes an inner cladding part having a refractive index that is constant in the radial direction, the inner cladding part being in contact with the core , and an outer cladding part surrounding the inner cladding part , a refractive index of a region of the inner core part in contact with the outer core part is higher than a refractive index of the outer core part , the refractive index of the outer core part is gradually decreased from an inner circumferential side to an outer circumferential side, and a refractive index of the inner cladding part is equal to a refractive index of an outermost circumferential part of the outer core part and is not greater than a refractive index of the outer cladding part , characterized in that when a distance from the center axis (C) to an internal circumference of the outer core part is defined as r1, a distance from the center axis (C) to an outer circumference of the outer core part is defined as rs, a relative refractive index difference of the outer core part to the outer cladding part at the distance r from the center axis (C) is defined as Δ(r), and the relative refractive index difference of the inner cladding part to the outer cladding part is defined as Δ2, S, which is expressed by the following Equation , is 0.3% μm or less: S = ∫ r 1 rs Δ r − Δ 2 dr a core ; and a cladding surrounding the core , wherein the core includes an inner core part including a center axis (C) of the core and having a refractive index that is constant in a radial direction, and an outer core part surrounding the inner core part , an inner core part including a center axis (C) of the core and having a refractive index that is constant in a radial direction, and an outer core part surrounding the inner core part , the cladding includes an inner cladding part having a refractive index that is constant in the radial direction, the inner cladding part being in contact with the core , and an outer cladding part surrounding the inner cladding part , an inner cladding part having a refractive index that is constant in the radial direction, the inner cladding part being in contact with the core , and an outer cladding part surrounding the inner cladding part , a refractive index of a region of the inner core part in contact with the outer core part is higher than a refractive index of the outer core part , the refractive index of the outer core part is gradually decreased from an inner circumferential side to an outer circumferential side, and a refractive index of the inner cladding part is equal to a refractive index of an outermost circumferential part of the outer core part and is not greater than a refractive index of the outer cladding part , characterized in that when a distance from the center axis (C) to an internal circumference of the outer core part is defined as r1, a distance from the center axis (C) to an outer circumference of the outer core part is defined as rs, a relative refractive index difference of the outer core part to the outer cladding part at the distance r from the center axis (C) is defined as Δ(r), and the relative refractive index difference of the inner cladding part to the outer cladding part is defined as Δ2, S, which is expressed by the following Equation , is 0.3% μm or less: S = ∫ r 1 rs Δ r − Δ 2 dr <EOS>