An optical fiber, which is a two-mode optical fiber for propagating an LP01 mode component and an LP11 mode component included in signal light, the optical fiber comprising: an inner core having an αth-power-type refractive index distribution and a maximum refractive index of n1; an outer core surrounding the inner core and having a refractive index ofnl'; and a clad surrounding the outer core and having a refractive index of n2 (n1' < n2 < n1), wherein a gradient dΔτ/dλ of a mode dispersion Δτ defined by the following formula: Δτ = 1 v g 11 − 1 v g 01 is |0.5| ps/km/nm or less in a predetermined wavelength band where: in the formula, v g 11 represents a group velocity of the LP11 mode component and v g 01 represents a group velocity of the LP01 mode component, and wherein the optical fiber satisfies: (Condition 1): Ra ≤ 0.7 (Condition 2): 0.1< Rd < 0.5 (Condition 3): 1.0 ≤ α ≤ 10.0, where: a radius of the inner core is set to rl; a radius of an outer circumference of the outer core is set to a; a relative refractive index difference between the outer core and the clad is set to Δ-; a relative refractive index difference between the inner core and the clad is set to Δ+; Ra = r 1 / a; and Rd = Δ − / Δ +. an inner core having an αth-power-type refractive index distribution and a maximum refractive index of n1; an outer core surrounding the inner core and having a refractive index ofnl'; and a clad surrounding the outer core and having a refractive index of n2 (n1' < n2 < n1), wherein a gradient dΔτ/dλ of a mode dispersion Δτ defined by the following formula: Δτ = 1 v g 11 − 1 v g 01 is |0.5| ps/km/nm or less in a predetermined wavelength band where: in the formula, v g 11 represents a group velocity of the LP11 mode component and v g 01 represents a group velocity of the LP01 mode component, and wherein the optical fiber satisfies: (Condition 1): Ra ≤ 0.7 (Condition 2): 0.1< Rd < 0.5 (Condition 3): 1.0 ≤ α ≤ 10.0, where: a radius of the inner core is set to rl; a radius of an outer circumference of the outer core is set to a; a relative refractive index difference between the outer core and the clad is set to Δ-; a relative refractive index difference between the inner core and the clad is set to Δ+; Ra = r 1 / a; and Rd = Δ − / Δ +. in the formula, v g 11 represents a group velocity of the LP11 mode component and v g 01 represents a group velocity of the LP01 mode component, and wherein the optical fiber satisfies: (Condition 1): Ra ≤ 0.7 (Condition 2): 0.1< Rd < 0.5 (Condition 3): 1.0 ≤ α ≤ 10.0, where: a radius of the inner core is set to rl; a radius of an outer circumference of the outer core is set to a; a relative refractive index difference between the outer core and the clad is set to Δ-; a relative refractive index difference between the inner core and the clad is set to Δ+; Ra = r 1 / a; and Rd = Δ − / Δ +. (Condition 1): Ra ≤ 0.7 (Condition 2): 0.1< Rd < 0.5 (Condition 3): 1.0 ≤ α ≤ 10.0, where: a radius of the inner core is set to rl; a radius of an outer circumference of the outer core is set to a; a relative refractive index difference between the outer core and the clad is set to Δ-; a relative refractive index difference between the inner core and the clad is set to Δ+; Ra = r 1 / a; and Rd = Δ − / Δ +. a radius of the inner core is set to rl; a radius of an outer circumference of the outer core is set to a; a relative refractive index difference between the outer core and the clad is set to Δ-; a relative refractive index difference between the inner core and the clad is set to Δ+; Ra = r 1 / a; and Rd = Δ − / Δ +.