A gear device includes a first gear, a second gear, a third gear, a housing, and a cover. The second gear is offset eccentrically when the first gear is rotated. The third gear is rotated at a speed slower than a rotation speed of the first gear when the second gear is offset eccentrically. The housing accommodates the first gear, the second gear, and the third gear. The cover is attached to the housing. The cover includes a deformation resistance portion that limits deformation of the cover in an axial direction of the third gear.