A gear device includes a first gear, a second gear, and a third gear, a stopper plate, and a housing. The second gear is offset eccentrically when the first gear is rotated. The third gear is rotated at a slower speed than the first gear when the second gear is offset eccentrically. The stopper plate is engaged with the second gear thereby restricting rotational movement of the second gear while permitting linear movement of the second gear. The housing accommodates the stopper plate in a state restricting rotational movement of the stopper plate while permitting linear movement of the stopper plate. The stopper plate includes an arm extending in a linear movement direction of the stopper plate. The housing includes an accommodation pit accommodating the arm. The arm includes a branching portion. The accommodation pit includes a projected wall arranged in the branching portion.