A method of designing an optical fiber, the method comprising: determining a wavelength of propagating light, the fiber transmission loss of a photonic crystal fiber, PCF, at the wavelength of propagating light and a Raman gain coefficient 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 and an interval of air holes of the PCF satisfying the A eff; calculating a bending loss in the fundamental mode at the minimum bending radius in the PCF having a structure calculated in the calculating of a diameter and an interval and calculating a bending loss in the fundamental mode at a propagation length from the propagation distance; checking that the bending loss at the propagation length is a predetermined value or less and determining the structure of the PCF calculated in the calculating of a diameter and an interval; and increasing the number of modes by one in a case where the bending loss of the propagation length is equal or larger than the predetermined value in the checking, and repeating the calculating of a diameter and an interval, the calculating of the bending loss, and the checking until the number of modes reaches the largest number n of propagation modes. [Numerical Expression 1] 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 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 [Numerical Expression 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 wavelength of propagating light, the fiber transmission loss of a photonic crystal fiber, PCF, at the wavelength of propagating light and a Raman gain coefficient 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 and an interval of air holes of the PCF satisfying the A eff; calculating a bending loss in the fundamental mode at the minimum bending radius in the PCF having a structure calculated in the calculating of a diameter and an interval and calculating a bending loss in the fundamental mode at a propagation length from the propagation distance; checking that the bending loss at the propagation length is a predetermined value or less and determining the structure of the PCF calculated in the calculating of a diameter and an interval; and increasing the number of modes by one in a case where the bending loss of the propagation length is equal or larger than the predetermined value in the checking, and repeating the calculating of a diameter and an interval, the calculating of the bending loss, and the checking until the number of modes reaches the largest number n of propagation modes. [Numerical Expression 1] 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 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 [Numerical Expression 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 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