A method of designing an optical fiber, the method comprising: determining a Raman gain coefficient of a photonic crystal fiber, PCF, a wavelength of propagating light, the transmission loss of the PCF, a beam quality M 2 after PCF propagation, a laser output power value, a propagation distance, and a minimum bending radius; calculating the largest number n of propagation modes that can be propagated by using Mathematical Formula 1; calculating an effective area A eff from the fiber transmission loss and the Raman gain coefficient by using Mathematical Formula 2; calculating a diameter d and an interval A of air holes of the PCF having the A eff or more and plotting first points having the A eff or more on a graph having a horizontal axis set as d/Λ and a vertical axis set as Λ, for a fundamental mode bending loss of 1 dB/m or less at the minimum bending radius in the wavelength of propagating light; calculating at the minimum bending radius for a minimum high-order mode cut off by the PCF based on the diameter d and the interval A of the air holes of the PCF, a high-order bending loss of a high-order mode in the wavelength of propagating light, and plotting second points having a high-order bending loss of the high-order mode of 1 dB/m or more on the graph having a horizontal axis set as d/Λ and a vertical axis set as A; and detecting an overlapping range in which an area of the first points plotted on the graph and an area of the second points plotted on the graph are overlapped, and determining a PCF structure having air holes of the diameter d and the interval Λ that are in the overlapping range. M 2 = Cn V cutoff n + 1, m 2 + 2 n 2 − 1 3 Herein, V cutoff n + 1, m: Cutoff Value of V Number in Immediately Upper Mode LP n+1,m Cn: Cn = 1 + 1 2 cos ξ When LP 1,m, Cn = 1 When LP n,m (n≠1) n: Allowable number of Propagation modes [Mathematical Formula 2] P th = 16 A eff g R L eff Herein, P th: SRS Threshold Value L eff: Effective Interaction Length L eff = 1 − exp − α p L α p α: Transmission Loss and α p = α 4.343 g R: Raman Gain Coefficient determining a Raman gain coefficient of a photonic crystal fiber, PCF, a wavelength of propagating light, the transmission loss of the PCF, a beam quality M 2 after PCF propagation, a laser output power value, a propagation distance, and a minimum bending radius; calculating the largest number n of propagation modes that can be propagated by using Mathematical Formula 1; calculating an effective area A eff from the fiber transmission loss and the Raman gain coefficient by using Mathematical Formula 2; calculating a diameter d and an interval A of air holes of the PCF having the A eff or more and plotting first points having the A eff or more on a graph having a horizontal axis set as d/Λ and a vertical axis set as Λ, for a fundamental mode bending loss of 1 dB/m or less at the minimum bending radius in the wavelength of propagating light; calculating at the minimum bending radius for a minimum high-order mode cut off by the PCF based on the diameter d and the interval A of the air holes of the PCF, a high-order bending loss of a high-order mode in the wavelength of propagating light, and plotting second points having a high-order bending loss of the high-order mode of 1 dB/m or more on the graph having a horizontal axis set as d/Λ and a vertical axis set as A; and detecting an overlapping range in which an area of the first points plotted on the graph and an area of the second points plotted on the graph are overlapped, and determining a PCF structure having air holes of the diameter d and the interval Λ that are in the overlapping range. M 2 = Cn V cutoff n + 1, m 2 + 2 n 2 − 1 3 Herein, V cutoff n + 1, m: Cutoff Value of V Number in Immediately Upper Mode LP n+1,m Cn: Cn = 1 + 1 2 cos ξ When LP 1,m, Cn = 1 When LP n,m (n≠1) n: Allowable number of Propagation modes [Mathematical Formula 2] P th = 16 A eff g R L eff Herein, P th: SRS Threshold Value L eff: Effective Interaction Length L eff = 1 − exp − α p L α p α: Transmission Loss and α p = α 4.343 g R: Raman Gain Coefficient V cutoff n + 1, m: Cutoff Value of V Number in Immediately Upper Mode LP n+1,m Cn: Cn = 1 + 1 2 cos ξ When LP 1,m, Cn = 1 When LP n,m (n≠1) n: Allowable number of Propagation modes P th: SRS Threshold Value L eff: Effective Interaction Length L eff = 1 − exp − α p L α p α: Transmission Loss and α p = α 4.343 g R: Raman Gain Coefficient