A cup gasket is prevented from coming out to the low-pressure fluid side. A rotating-side seal ring 5 is formed in a substantially pentagonal shape in cross section having a cut portion 5a at which a corner portion on the inside-diameter side and the side opposite to a sealing face is cut in a tapered shape. A cup gasket 7 is fitted over the inside-diameter side and the back-surface side of the rotating-side seal ring 5, and is formed in a substantially L shape in cross section including an axial portion 7a in contact with the inside-diameter side of the rotating-side seal ring 5 and an inner cylindrical portion of the sleeve 2 and a radial portion 7b in contact with the back-surface side of the rotating-side seal ring 5 and a radial portion of the sleeve 2, and has a corner portion 7c facing the cut portion 5a formed in a shape along the cut portion 5a. The thickness of the corner portion 7c of the cup gasket 7 is set larger than a gap between the inside-diameter side of the rotating-side seal ring 5 and the inner cylindrical portion of the sleeve 2.