A gear device includes an internal gear and an external gear that meshes with the internal gear, in which the internal gear is made of a resin, the external gear is made of a material having a smaller linear expansion coefficient than the resin, and PCDs of the internal gear and the external gear before use of the gear device are set such that lost motion becomes 0 min or more and 15 min or less due to a difference in a thermal expansion amount between the internal gear and the external gear at a time of use of the gear device.