A stress-strain relation simulation method comprising: an experimental-value acquisition step of plastically deforming an elastic-plastic material to acquire experimental values of a stress-strain relation; a first computer implemented material-constant identification step, wherein an elastic-plastic constitutive model is defined as a function of stress and back stress, and wherein a kinematic-hardening incremental vector dα ij of a yield surface as defined in Equation and the experimental values acquired in the experimental-value acquisition step are used in order to identify the material constants Y, C, B, R sat, b, m, h contained in the Yoshida-Uemori model; a computer implemented re-identification step using the experimental values of the stress-strain relation acquired in the experimental-value acquisition step to re-identify the yield stress as being the stress when the tangential gradient dσ/dε of stress versus strain starts to decrease; a second computer implemented material-constant identification step of identifying the material constants C c, A 1, A 2, n 1, n 2 contained in Equation and that determine characteristics immediately after the stress reversal, based on Equation into which the material constants identified at the first material-constant identification step are substituted; and a step of, by the computer, simulating the stress-strain relation of the elastic-plastic material using Equation and Equation into which the material constants identified are substituted, d α ij = C a Y σ ij − α ij − C 0 ρ X ij d ε p eq C = C 0 + C C exp − X eq n A where a: maximum value of movement of yield surface Y: yield stress α ij: movement vector of yield surface σ ij: stress vector X ij: yield - surface kinematic - hardening amount after stress reversal (X eq is equivalent value thereof) d ε p eq: equivalent plastic - strain increment C 0, C c, A, n: material constants and wherein variables X ij, p, A, and n in the Equations and are represented by Equation. If σ eq ≥ σ e ⁢ q max, σ eq = σ e ⁢ q max X ij = α ij, ρ = 1, A = A 1, n = n 1 If σ eq < σ e ⁢ q max X ij = α tmp ij − α ij, ρ = 1 2, A = A 2, n = n 2} where σ eq max: maximum value of equivalent stress when isotropic hardening is assumed α tmp ij: back stress at the time of stress reversal A 1, A 2, n 1, n 2: material constants an experimental-value acquisition step of plastically deforming an elastic-plastic material to acquire experimental values of a stress-strain relation; a first computer implemented material-constant identification step, wherein an elastic-plastic constitutive model is defined as a function of stress and back stress, and wherein a kinematic-hardening incremental vector dα ij of a yield surface as defined in Equation and the experimental values acquired in the experimental-value acquisition step are used in order to identify the material constants Y, C, B, R sat, b, m, h contained in the Yoshida-Uemori model; a computer implemented re-identification step using the experimental values of the stress-strain relation acquired in the experimental-value acquisition step to re-identify the yield stress as being the stress when the tangential gradient dσ/dε of stress versus strain starts to decrease; a second computer implemented material-constant identification step of identifying the material constants C c, A 1, A 2, n 1, n 2 contained in Equation and that determine characteristics immediately after the stress reversal, based on Equation into which the material constants identified at the first material-constant identification step are substituted; and a step of, by the computer, simulating the stress-strain relation of the elastic-plastic material using Equation and Equation into which the material constants identified are substituted, d α ij = C a Y σ ij − α ij − C 0 ρ X ij d ε p eq C = C 0 + C C exp − X eq n A where a: maximum value of movement of yield surface Y: yield stress α ij: movement vector of yield surface σ ij: stress vector X ij: yield - surface kinematic - hardening amount after stress reversal (X eq is equivalent value thereof) d ε p eq: equivalent plastic - strain increment C 0, C c, A, n: material constants a: maximum value of movement of yield surface Y: yield stress α ij: movement vector of yield surface σ ij: stress vector X ij: yield - surface kinematic - hardening amount after stress reversal (X eq is equivalent value thereof) d ε p eq: equivalent plastic - strain increment C 0, C c, A, n: material constants σ eq max: maximum value of equivalent stress when isotropic hardening is assumed α tmp ij: back stress at the time of stress reversal A 1, A 2, n 1, n 2: material constants