A thin metal plate such as detent spring has the shape deformation due to the phenomenon of spring back after press machining and heat treatment process. This requires the correction of spring shape and force in final inspection process. To do correction of the shape deformation the impact force is manually applied to the bended part of detent spring after measuring the shape deformation and spring force. To develop the automatic spring force correction system, applied force of occurring plastic deformation must be derived from the experimental method. But frequent change of spring shape and material makes it difficult to accomplish the experimental method to be applied. This paper describes the analytical method for detent spring force correction system is to be substituted for the experimental method. FEM(Finite Element Method) is used to find the boundary value between elastic and plastic deformation in the analytical method. To confirm the validity of the analytical method, the result of two methods is compared each other at various applied force conditions. It shows that the simulation result of the analytical method is consistent with the result of the experimental method within the error bound ${pm}$5%. The result of this paper is useful for development of the automatic spring correction system and reduction of the complicated and tedious processes involved in experimental method.