Provided is a relative pressure control system has a simple configuration, but enables accurate regulation of a division ratio of an operation gas, and concurrently makes it possible to securely drain the operation gas from an operation gas pipeline in case of emergency. The system includes a plurality of air operated valves of a normally open type that are connected to an operation gas pipeline supplied with an operation gas; pressure sensors that are series connected to the respective air operated valves and that detect output pressures of the respective air operated valves; a controller that controls operation pressures of the respective air operated valves in accordance with the pressures detected by the pressure sensors; and a hard-interlock solenoid valve that correlates the plurality of air operated valves to one another so that at least one of the plurality of air operated valves is normally opened. In the configuration, an opening of a specified one of the plurality of air operated valves is regulated, the operation gas is output at a predetermined division ratio.