A planetary gear reducer has a planetary carrier supported by an input-side main bearing and an output-side main bearing. An angular contact ball bearing or a deep groove ball bearing which requires less installation space and is inexpensive is used for the input-side main bearing. A tapered roller bearing having a large capacity is used for the output-side main bearing. A compact, flat and inexpensive planetary gear reducer can be realized in comparison with a case in which the both main bearings are tapered roller bearings.