A gear device includes an internal gear, an external gear, and a wave generator. The external gear includes a cylinder section, a diaphragm, and a boss section. The diaphragm includes a first coupling section, a second coupling section, and a diaphragm main body. A ratio of the length from one opening end of the cylinder section to the surface of the diaphragm on the opposite side of the opening end in a rotation axis direction to the length from the inner circumferential surface of the boss section coupled to the second coupling section to the outer circumferential surface of the cylinder section in the radial direction is 1.0 or more and 5.0 or less, and t(C)≥t(A), where t(A) represents the thickness of the inner circumferential end of the diaphragm and t(C) represents the thickness of the outer circumferential end.