A planetary reduction gear using a helical gear is configured such that a rear-stage sun gear is rotatably supported by a device housing via a radial bearing on one side in the direction of a center axis, and is rotatably supported by a rear-stage planetary carrier via a thrust bearing on the other side. The rear-stage planetary carrier is rotatably supported by the device housing via a rear-stage carrier bearing. A preload is applied to the thrust bearing by a preload mechanism mounted to the rear-stage planetary carrier. Displacement of the rear-stage sun gear caused by a thrust force generated due to engagement with a rear-stage planetary gear can be suppressed, and an angle error between input rotation and output rotation can be suppressed.