A method of evaluating a residual stress, comprising: a condition setting step (S21, S91) of setting a processing condition of water jet peening for a processing target; a flow analysis step (S23, S93) of numerically analyzing, using computational fluid dynamics, a jet flow when a fluid is injected from a nozzle model to a processing target model in accordance with the processing condition, and obtaining a void fraction (f) which is a volume fraction of bubbles contained in a unit volume of the fluid, and a collapse fraction (η), which is a volume fraction of the bubbles which collapse in a unit time in the unit volume of the fluid, at each position on a surface of the processing target model; an impact pressure correlation value calculating step (S24, S94) of obtaining, at each position on the surface of the processing target model, an impact pressure correlation value (Pc), which is a product of the void fraction and the collapse fraction at each position; an experimental value acquisition step (S25, S95) of obtaining, at multiple positions of the surface of the processing target model, an impact pressure experimental value (Pr), which is an experimental value of an impact pressure applied to a surface of the processing target due to the water jet peening under the processing condition; a prediction step (S26, S96) of obtaining an impact pressure predicted value (Pp), which is a predicted value of the impact pressure applied to each position on the surface of the processing target due to the water jet peening under the processing condition, by determining a coefficient k for associating the impact pressure correlation value (Pc) at each position on the surface of the processing target model with the impact pressure experimental value (Pr) applied to the surface of the processing target by the water jet peening under the processing condition, wherein the coefficient k is obtained so that the difference between the position on the surface of the processing target where the impact pressure experimental value Pr is obtained and the impact pressure correlation value Pc corresponding to that position fall within a predetermined range, wherein the impact pressure predicted value (Pp) is equal to the coefficient k times the impact pressure correlation value (Pc); and a residual-stress evaluation step (S27, S97) of calculating, by performing a finite element method analysis, a residual stress of the processing target after the water jet peening under the processing condition by using the impact pressure predicted value obtained in the prediction step as an input condition. a condition setting step (S21, S91) of setting a processing condition of water jet peening for a processing target; a flow analysis step (S23, S93) of numerically analyzing, using computational fluid dynamics, a jet flow when a fluid is injected from a nozzle model to a processing target model in accordance with the processing condition, and obtaining a void fraction (f) which is a volume fraction of bubbles contained in a unit volume of the fluid, and a collapse fraction (η), which is a volume fraction of the bubbles which collapse in a unit time in the unit volume of the fluid, at each position on a surface of the processing target model; an impact pressure correlation value calculating step (S24, S94) of obtaining, at each position on the surface of the processing target model, an impact pressure correlation value (Pc), which is a product of the void fraction and the collapse fraction at each position; an experimental value acquisition step (S25, S95) of obtaining, at multiple positions of the surface of the processing target model, an impact pressure experimental value (Pr), which is an experimental value of an impact pressure applied to a surface of the processing target due to the water jet peening under the processing condition; a prediction step (S26, S96) of obtaining an impact pressure predicted value (Pp), which is a predicted value of the impact pressure applied to each position on the surface of the processing target due to the water jet peening under the processing condition, by determining a coefficient k for associating the impact pressure correlation value (Pc) at each position on the surface of the processing target model with the impact pressure experimental value (Pr) applied to the surface of the processing target by the water jet peening under the processing condition, wherein the coefficient k is obtained so that the difference between the position on the surface of the processing target where the impact pressure experimental value Pr is obtained and the impact pressure correlation value Pc corresponding to that position fall within a predetermined range, wherein the impact pressure predicted value (Pp) is equal to the coefficient k times the impact pressure correlation value (Pc); and a residual-stress evaluation step (S27, S97) of calculating, by performing a finite element method analysis, a residual stress of the processing target after the water jet peening under the processing condition by using the impact pressure predicted value obtained in the prediction step as an input condition.