A gear device, comprising: a gear mechanism including at least a first gear and a second gear; a case configured to rotatably accommodate the gear mechanism and formed from a material having an expansion coefficient that differs from that of the gear mechanism; a first restriction that restricts a position of the first gear relative to the case in a gear radial direction of the first gear; and a second restriction that restricts a position of a second gear relative to the case in a gear radial direction of the second gear, wherein the first restriction includes: a first case projection formed on the case; and a first gear projection formed on the first gear and supporting the first case projection from an outer side in the gear radial direction of the first gear to restrict positions of the first gear and the case, and a first preset gap extends between the first gear projection and the first case projection the expansion coefficient of the material forming the case is smaller than that of the gear mechanism thereby resulting in that, the first preset gap is relatively small when the ambient temperature during usage is low and relatively large when the ambient temperature during usage is high, and characterized in that the second restriction includes: a second gear projection formed on the second gear; and a second case projection formed on the case and supporting the second gear projection from an outer side in the gear radial direction of the second gear to restrict positions of the second gear and the case, and a second preset gap extends between the second gear projection and the second case projection, and the second preset gap is set to be relatively large when the ambient temperature during usage is low, and relatively small when the ambient temperature during usage is high. a gear mechanism including at least a first gear and a second gear; a case configured to rotatably accommodate the gear mechanism and formed from a material having an expansion coefficient that differs from that of the gear mechanism; a first restriction that restricts a position of the first gear relative to the case in a gear radial direction of the first gear; and a second restriction that restricts a position of a second gear relative to the case in a gear radial direction of the second gear, wherein the first restriction includes: a first case projection formed on the case; and a first gear projection formed on the first gear and supporting the first case projection from an outer side in the gear radial direction of the first gear to restrict positions of the first gear and the case, and a first preset gap extends between the first gear projection and the first case projection the expansion coefficient of the material forming the case is smaller than that of the gear mechanism thereby resulting in that, the first preset gap is relatively small when the ambient temperature during usage is low and relatively large when the ambient temperature during usage is high, and characterized in that the second restriction includes: a second gear projection formed on the second gear; and a second case projection formed on the case and supporting the second gear projection from an outer side in the gear radial direction of the second gear to restrict positions of the second gear and the case, and a second preset gap extends between the second gear projection and the second case projection, and the second preset gap is set to be relatively large when the ambient temperature during usage is low, and relatively small when the ambient temperature during usage is high. a first case projection formed on the case; and a first gear projection formed on the first gear and supporting the first case projection from an outer side in the gear radial direction of the first gear to restrict positions of the first gear and the case, and a first preset gap extends between the first gear projection and the first case projection the expansion coefficient of the material forming the case is smaller than that of the gear mechanism thereby resulting in that, the first preset gap is relatively small when the ambient temperature during usage is low and relatively large when the ambient temperature during usage is high, and characterized in that the second restriction includes: a second gear projection formed on the second gear; and a second case projection formed on the case and supporting the second gear projection from an outer side in the gear radial direction of the second gear to restrict positions of the second gear and the case, and a second preset gap extends between the second gear projection and the second case projection, and the second preset gap is set to be relatively large when the ambient temperature during usage is low, and relatively small when the ambient temperature during usage is high. a second gear projection formed on the second gear; and a second case projection formed on the case and supporting the second gear projection from an outer side in the gear radial direction of the second gear to restrict positions of the second gear and the case, and a second preset gap extends between the second gear projection and the second case projection, and the second preset gap is set to be relatively large when the ambient temperature during usage is low, and relatively small when the ambient temperature during usage is high.