A composite seal ring has a resin seal ring and a metal spring. The resin seal ring is attached in an outer peripheral groove of a valve that opens and closes a passage by a rotational displacement, and the resin seal ring seals between an inner wall of a passage portion that forms the passage and the valve, in the fully closed state of the valve. A metal spring is circularly disposed within a radial width of the resin seal ring. The spring is configured to press the resin seal ring radially outwardly and radially inwardly due to the pressure acting on the resin seal ring or a deformation of the resin seal ring.