When a first output shaft (5) of a planetary differential reduction device (1) is driven to rotate, a first internal gear (24) of a first planetary reduction part (20) and a second internal gear (34) of a second planetary reduction part (30) are rotated in the same direction at the same speed, and a planetary carrier (63) of a third planetary reduction part (40) is also rotated in the same direction at the same speed, whereby a second output shaft (6) connected to the planetary carrier (63) is rotated synchronously with the first output shaft (5). When differential rotation is inputted to the second sun gear (31), a reduced rotational output is output to the planetary carrier (63) of the third planetary reduction part (40). The second output shaft (6) is added with the reduced rotational output for differential rotation, whereby the second output shaft (6) is shifted to a differential rotating state. It is possible to realize a small, compact and highly reliable differential mechanism compared to a case in which spur gears and bevel gears are used.