Recently, regulation of ventilator installation and its details has been revised and the establishment of heat recovery ventilator in newly built apartments has been obligated. This study was done to offer the method of operation and design of heat recovery ventilator to save energy by measuring its efficiency and comparing with the results of experiment. This paper confirmed that it is desirable to operate heat recovery ventilator by using "by-pass mode" within $60{sim}80%$ scope of the difference indoor absolute humidity in spring and autumn and outdoor absolute humidity and heat recovery ventilator of energy saving effect is better than constant air volume system.