To lessen an impact in fully opening or fully closing an open/close valve.

The open/close valve includes: a valve casing having a valve seat; a valve stem 18 rotatably disposed to the inside of the valve casing; a valve element supported by the valve stem 18 and seated on/separated from the valve seat; a return spring 21 urging an elastic force to the valve stem 18; and an actuator rotating the valve stem 18. A buffer spring 23 accumulating power by the actuator in seating or separation of the valve element is interposed between the return spring 21 and the actuator. One end of the buffer spring 23 is locked to the return spring 21, and is locked to a lever 22 connected with the actuator, and the other end thereof is locked to the buffer lever 24 supported by the valve stem 18.

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