A plurality of concave portion to be spaced apart from each other in a circumferential direction through a pillar portion is formed in an inner circumferential side of a side surface of a seal ring mounted in a shaft groove formed in an outer circumferential surface of a shaft. The concave portion including a deepest portion, and two inclined portions, connects the inclined portion positioned at an opposite side in a rotational direction and the adjacent pillar portion with a curved surface having a shape convex, and further, makes a width in the circumferential direction of the inclined portion of the rotational direction side smaller than a width in the circumferential direction of the inclined portion of the opposite side in the rotational direction.