To reduce sliding resistance which may act on a planetary gear in revolution of the planetary gear.

Thrust force F2 to move a planetary gear 510 to a first plate material 561 side, acts on a large-diameter portion 511 of the planetary gear 510. Further, thrust force F3 to move the planetary gear 510 in the direction separating from the first plate material 561 acts on the planetary gear 510 by engagement of a small-diameter portion 512 of the planetary gear 510 with an internal gear 520. The thrust force F3 acting on the small-diameter portion 512 of the planetary gear 510 is larger than the thrust force F2 acting on the large-diameter portion 511 of the planetary gear 510. As a result, the thrust force indicated by an arrow F5 in a drawing acts on the planetary gear 510, and the planetary gear 510 is pressed to a rotary disk 540 side.