An optical fiber design method for designing a photonic crystal fiber, wherein the photonic crystal fiber has a core region and a cladding region, the cladding region having a plurality of holes arranged in the optical fiber along a longitudinal direction, wherein in the cladding region in a cross section a hole ratio which is an area of the holes per unit area is larger in a central side than in an outer side in a portion corresponding to the cladding region, wherein an interval Λ among all of the holes is constant and a diameter d1 of the holes is larger for holes adjacent to the core region than a diameter d for holes farther from the core region, the method characterized by: Defining a specification value, the specification value including a wavelength of light propagated in the photonic crystal fiber, power P th of the light propagated in the photonic crystal fiber, and a propagation distance L through which the light is propagated in the photonic crystal fiber; calculating a required effective cross-sectional area A eff of the photonic crystal fiber by utilizing a mathematical formula C5 P th = 16 A eff g R L on the basis of the power P th and the propagation distance L defined in the specification value and on the basis of a Raman gain coefficient g R; calculating by numerical calculation analysis in a design range of diameters d and intervals A of the holes an effective cross-sectional area A eff from the diameter d and the interval A of the holes of the photonic crystal fiber, determining intervals A and ratios (dl/d) under which the effective cross-sectional area is equal to or exceeds the required effective cross-sectional area A eff, plotting the A eff on a graph representing the interval Λ of the holes on a horizontal axis and a ratio (dl/d) of the diameter d1 of the holes adjacent to the core region and the diameter d of the holes farther away from the core region on a vertical axis, calculating intervals A and ratios (d1/d) under which a loss with respect to the LP02 mode is 0.1 dB/m, and plotting the calculated intervals Λ and ratios (d1/d) on the graph; calculating intervals A and ratios (dl/d) under which a bending loss in the basic mode is 1 dB/km at a predefined bending radius, and plotting the calculated intervals Λ and ratios (dl/d) on the graph; detecting an overlapping region of intervals Λ and ratios (d1/d) in the graph, in which the effective cross-sectional area is equal to or exceeds the required effective cross-sectional area A eff, the loss with respect to the LP02 mode is 0.1 dB/m or more, and the bending loss in the basic mode is 1 dB/km or less at the predefined bending radius; and determining the interval Λ and the ratio (d1/d) in the overlapping region as a structure of the photonic crystal fiber. Defining a specification value, the specification value including a wavelength of light propagated in the photonic crystal fiber, power P th of the light propagated in the photonic crystal fiber, and a propagation distance L through which the light is propagated in the photonic crystal fiber; calculating a required effective cross-sectional area A eff of the photonic crystal fiber by utilizing a mathematical formula C5 P th = 16 A eff g R L on the basis of the power P th and the propagation distance L defined in the specification value and on the basis of a Raman gain coefficient g R; calculating by numerical calculation analysis in a design range of diameters d and intervals A of the holes an effective cross-sectional area A eff from the diameter d and the interval A of the holes of the photonic crystal fiber, determining intervals A and ratios (dl/d) under which the effective cross-sectional area is equal to or exceeds the required effective cross-sectional area A eff, plotting the A eff on a graph representing the interval Λ of the holes on a horizontal axis and a ratio (dl/d) of the diameter d1 of the holes adjacent to the core region and the diameter d of the holes farther away from the core region on a vertical axis, calculating intervals A and ratios (d1/d) under which a loss with respect to the LP02 mode is 0.1 dB/m, and plotting the calculated intervals Λ and ratios (d1/d) on the graph; calculating intervals A and ratios (dl/d) under which a bending loss in the basic mode is 1 dB/km at a predefined bending radius, and plotting the calculated intervals Λ and ratios (dl/d) on the graph; detecting an overlapping region of intervals Λ and ratios (d1/d) in the graph, in which the effective cross-sectional area is equal to or exceeds the required effective cross-sectional area A eff, the loss with respect to the LP02 mode is 0.1 dB/m or more, and the bending loss in the basic mode is 1 dB/km or less at the predefined bending radius; and determining the interval Λ and the ratio (d1/d) in the overlapping region as a structure of the photonic crystal fiber.