PURPOSE: To absorb the impact during the engagement of a ring gear with a planetary gear by facilitating the assembly of the planetary gear of low vibration an low noise type.

CONSTITUTION: The inner circumferential surface of a casing to constitute a planetary gear device is polygonal, and an elastic material 18 is fixed to the outer circumferential surface and each end faces of a ring gear 17. The ring gear 17 is inserted into the inner circumferential surface of a first easing 19a to avoid the direct contact of the ring gear 17 with the easing, and the impact during the engagement of the ring gear with the planetary gear is dispersively absorbed by the elastic material 18, and little vibration generated by the impact is transmitted to the easing. The ring gear is of a simple structure only for insertion, and the assembly of the device is easy.