An exhausting method includes determining an exhaust flow rate of a process gas to be a predetermined value that is less than or equal to a gas flow rate corresponding to a maximum process capability of a purification system when the process gas is diluted to a lower explosive limit; calculating a pressure drop amount per unit time to maintain the determined exhaust flow rate of the process gas, based on a relation between the exhaust flow rate and the pressure drop amount per unit time; and evacuating an inside of the chamber to maintain the determined exhaust flow rate, while controlling the pressure through an automatic pressure control valve by setting a target pressure value to be updated as a control value of the automatic pressure control valve at every predetermined time interval so as to achieve a calculated pressure drop amount per unit time.