This paper deals with the optimization design process of an interior-type permanent magnet synchronous motor (IPMSM) for electric compressors of air conditioners applied to electric cars and hybrid vehicles. Based on a prototype IPMSM, we identified the current level for efficiency and cogging torque using finite-element analysis (FEA). The IPMSM’s main design variables are optimized to primarily satisfy the target of efficiency and cogging torque. A second optimization design is carried out using the response surface method (RSM) for the main design variables obtained from the Taguchi method analysis. As a result, the optimization design is carried out through a two-step optimization design process using the Taguchi method and RSM. The IPMSMs are analyzed using FEA, and their validity is confirmed by comparing the test results against each step-by-step model. We validate an effective optimization design process using a two-stage design process. We conclude that the proposed approach provides an effective design method for engineers.