A fixed-side ring plate rotation regulation section is provided at an outer circumferential edge of a fixed-side ring plate, and an outer cylinder rotation regulation section is provided at an outer circumferential edge of an outer cylinder at a position corresponding to the fixed-side ring plate rotation regulation section. The fixed-side ring plate and the outer cylinder formed of different members from each other are fit to each other such that the fixed-side ring plate rotation regulation section and the outer cylinder rotation regulation section are in contact with each other so as not to rotate about a rotation axis thereof.