A member joining structure for a differential device, the member joining structure includes a ring gear including a helical tooth extending in a rotation axis direction; a differential case that is rotated with the ring gear; and a first support that is disposed in a first position at a back surface corresponding to one end of a rotation axis of the ring gear and that connects the differential case and the ring gear, and a second support that is disposed in a second position at a back surface corresponding to another end in the rotation axis direction of the ring gear and that connects the differential case and the ring gear; wherein the first support and the second support are spaced apart from each other with a cavity therebetween.