This paper presents a novel methodology for improving the reliability of electromagnetic devices and machines based on the reliability-based design optimization method. To achieve this, the method includes reliability analysis and optimization process taking into account uncertainties of design variables. One of the first-order reliability analysis techniques, called reliability index approach, is adopted to evaluate the reliability of performance functions with respect to probabilistic design variables. The proposed method has been successfully applied to designing a superconducting magnetic energy storage system. For verifying the efficiency and accuracy of the method, the results are compared with those of conventional optimization methods.