A method for controlling flow rates of outdoor air supplied to a room and room air discharged to an outside of the room in ventilation. The method of the present invention includes the steps of supplying outdoor air drawn through an air supply duct to a room at a first flow rate, and discharging room air drawn through an air discharge duct to an outside of the room at a second flow rate higher than the first flow rate. The first and second flow rates vary with rotation speeds of fans respectively mounted on an air supply duct and an air discharge duct.