A gas feeding duct, a suction duct, a first valve that is installed in the gas feeding duct and opens a flow passage only when gas feeding is underway, and a second valve that is installed in a part of the flow passage on an upstream side relative to the first valve, and opens the flow passage K along with gas feeding and closes the flow passage at least when suction is underway are included, and when neither gas feeding nor gas suctioning is underway, the second valve closes the flow passage in conjunction with the first valve unexpectedly opening the flow passage as a result of pressure variation in a part of the gas feeding duct on a downstream side relative to the first valve and thereby prevents reverse flow of a fluid to the upstream side relative to the second valve.