A seal ring mounting portion has: an outer circumferential surface portion that is provided so as to come into close contact with an inner circumferential portion of a seal ring, and extends along a circumferential direction of the seal ring in a state of being in close contact with the inner circumferential portion of the seal ring; and a holding portion that has a first wall portion provided on one width-direction side of the outer circumferential surface portion and a second wall portion provided on another width-direction side of the outer circumferential surface portion, the holding portion sandwiching and holding the seal ring from two sides in a width direction of the seal ring by the first wall portion and the second wall portion in the state of the seal ring being in close contact with the outer circumferential surface portion. The first wall portion and the second wall portion are provided so as to not overlap each other in a view from a central axis direction of the seal ring in a state of being mounted to the seal ring mounting portion.