PURPOSE: To make it possible to lubricate a ring gear repeatedly by a lubricating gear and to prevent wear of the ring gear and a drive gear by providing a spring roller, the lubricating gear, a spring, a stopper, etc., which are specified respectively.

CONSTITUTION: A spring roller 5 coming into rolling contact with a circumferential surface being concentric with a ring gear 1 driven to rotate by a drive gear is provided and a lubricating gear 6 is joined rotatably on the same axis as the spring roller 5, while a working part 6B in which lubricating teeth 67 engaging with the ring gear 1 are formed and a non-working part 6A in which the lubricating teeth 67 are not formed are provided on the outer circumference of the lubricating gear 6. Moreover, springs 52 pressing the lubricating teeth 67 against the tooth flank of the ring gear 1 on the side on which it engages with the drive gear are interposed between the lubricating gear 6 and the spring roller 5, and a stopper 8 which stops the rotation of the lubricating gear 6 in the range of rotation of the spring roller 5 in which the non-working part 6A of the lubricating gear 6 faces the ring gear 1, and deforms the springs 52 elastically is provided.