A planetary reduction gear device has first and second planetary gears meshed with a sun gear and an internal at different locations in a direction of an axis line; and a carrier assembly for supporting the first and second planetary gears. The carrier assembly has a first planetary carrier for supporting the first planetary gears and a second planetary carrier for supporting the second planetary gears. The first and second planetary carriers are fixed coaxially with each other in the direction of the axis line so as to rotate integrally. A planetary reduction gear device having a large torque capacity can be obtained.