The optimum condition for the cyclic steady state is of central importance in the design and operation of a pressure-swing adsorption (PSA) process. Numerical simulation by use of a mathematical model of the process is suitable for optimizing the operating condition. This paper describes the direct method for determining the cyclic steady state based on the mathematical model and the optimizing method for the PSA process. The proposed method was proved through application to a laboratory-scale PSA system for air separation. The results showed that the optimum operating condition was obtained by the proposed method in a short calculation time.