To operate a force feed system, in the event of a trouble in one of juxtaposed pressure vessels of the force feed system, only by use of the other pressure vessel.

The force feed system comprises a first flow passage provided between a first pressure chamber of a first force feed device and a first output port of a selector valve; a second flow passage provided between a first pressure chamber of a second force feed device and a second output port of the selector valve; a third flow passage provided between a second pressure chamber of the first force feed device and a second pressure chamber of the second force feed device; a fist bypass passage for connecting the first output port to the third flow passage; and a second bypass passage for connecting the second output port to the third flow passage. The connection of the third flow passage to the first output port through the first bypass passage is performed while stopping the first force feed device, and the connection of the third flow passage to the output port through the second bypass passage is performed while stopping the second force feed device.