In a seat component received in a receiving region of a first body of a body, one surface of a contacting portion and one surface of an engaging portion are planar and are flush with each other, and the other surface of the contacting portion and the other surface of the engaging portion are planar and are flush with each other. The engaging portion engages two portions of the first body, which form one opening of the receiving region, in a state where a distal end of the seat component reaches another opening of the receiving region. Furthermore, in the state where the engaging portion engages the portions of the first body, a second body of the body covers the one opening and contacts the engaging portion. Thereby, the engaging portion engages the three portions of the body, so that movement of the seat component is effectively limited.