Abstract: Paired planetary gears 3A, 3B are each provided on one end thereof with a long gear portion 31, on the other end with a short gear portion 32, and a neck portion 33 between the long gear portion 31 and the short gear portion 32. The long gear portion 31 of the planetary gear 3A is in engagement with the short gear portion 32 of the other planetary gear 3B at an inner gear portion 31b and with a gear portion 2a of a side gear 2A at an outer gear portion 31a. The long gear portion 31 of the other planetary gear 3B is in engagement with the short gear portion 32 of the planetary gear 3A at the inner gear portion 31b and with a gear portion 2b of a side gear 2B at the outer gear portion 31a.