A drive shaft reduction gear using a planetary gear mechanism and having a high durability, wherein a rotation of a drive shaft is transmitted to a wheel hub via the planetary gear mechanism, a ring gear hub provided in a first ring gear in the planetary gear mechanism is hooked to a bearing holder attached to an axle housing via a split ring, thereby a movement in an axial direction of the first ring gear is regulated, and further, a shaft portion provided in the ring gear hub is non-rotatably supported by an inner peripheral surface of a shaft end portion of the axle housing.