PURPOSE: To enable a large reduction gear ratio to be obtained by a simple structure by using a planetary gear speed reducer as the first stage and a differential planetary gear speed reducer as the second stage, and integrating the output shaft of the first stage speed reducer and the input shaft of the second stage speed reducer as an intermediate shaft.

CONSTITUTION: A planetary gear speed reducer comprises a sun gear 3a, a planetary gear 4a, and a fixed internal gear 5a. A differential planetary gear speed reducer comprises a sun gear 13a, a planetary gear 14a, and a driving internal gear 16a. The output shaft of the planetary gear speed reducer and the input shaft of the differential planetary gear speed reducer are integrated as an intermediate shaft 8 a thus, a large reduction gear ratio can be obtained by a simple structure.