This paper deals with the design of safety stock level for satisfying the required service rate of a production inventory system with production capacity constraint. After reviewing the previous literatures, this paper proposes a method to design the safety stock level. The proposed method based on the secant method for solving non-linear equations utilizes the simulation for estimating a service rate and the numerical analysis for calculating quantiles. Examples show the validity of the proposed method, and the influence of the service rate and production capacity upon the safety stock level is analyzed with the proposed method.