Provided is a flow rate ratio control device that even in change periods during which the actual flow rates of component gases change along with a change in the target total flow rate or target mixing ratio of mixed gas, can keep the actual total flow rate or actual mixing ratio of the mixed gas constant. Target flow rates set in flow rate control devices have continuous change periods during which target flow rate values continuously change from pre-change to post-change target flow rate values, respectively and correspondingly; and the time lengths of the continuous change periods are set to at least a followable time during which a flow rate control device of which a response speed to an actual component fluid flow rate is lowest allows the actual component fluid flow rate to substantially follow the target flow rate in the continuous change periods.