Recently, buildings with thermal energy storage system have increased because of its economic advantage. In thermal energy storage system using introduced outdoor air, building masses are used as thermal storage media and cooled by outdoor air introduced by a mechanical fan in night-time. In spring and autumn, the cooled building masses can work to reduce cooling load in day-time. In this study, the stored heat was calculated by using simple heat transfer models for various building masses, and the energy performance of this system was evaluated quantitatively by system simulation. It was found that the optimized flow rate of outdoor air can be obtained by comparing the electric power consumption by a mechanical fan with the reduced cooling load because the mechanical fan is the only device requires electric power in night-time.