An eccentric-rotation type planetary differential reduction gear speed reducer (10) includes a cylindrical housing (14). The cylindrical housing (14) is fixed to a first member (3) constituting a link of a robot, and an output shaft (16) of the speed reducer (10) is fixed to a second member (4) constituting a link of the robot. An eccentric shaft (11) constituting an input shaft of the speed reducer (10) is coupled to a motor shaft (2) of a servo motor mounted on the first member (3). Splines are formed directly on the outer peripheral surface of the motor shaft (2) to form a spline shaft (5). The spline shaft is fitted into a spline hole formed in the eccentric shaft (11), so that the motor shaft (2) is directly coupled to the eccentric shaft (11).