To provide a gear device with good airtightness by surely preventing foreign matter such as abrasion powder generated in the gear device from entering into a seal device, even in an strict operational specification or a severe environment.

The gear device comprises: an internal gear; an external gear meshed with the internal gear; a crankshaft engaged with the external gear and eccentrically oscillating the external gear; a support body rotatably supporting the crankshaft; a pair of bearings disposed between an inner peripheral face of the internal gear and an outer peripheral face of the support body and disposed on both sides in an axial direction of the external gear; and the seal device disposed between the inner peripheral face of the internal gear and the outer peripheral face of the support body, and arranged axially outside at least one bearing of the pair of the bearings. The internal gear and the support body are relatively rotated. A plate 136 is arranged on one bearing 113 side of the seal device 115, a space constituted of the seal device and the plate is filled with high-viscosity grease 160 different in property from lubricant lubricating the gear device.