Gear device and robot

A gear device comprising:

• an internal gear;
• an external gear having flexibility and configured to partially mesh with the internal gear and rotate around a rotation axis relatively to the internal gear;
• and
• a wave generator configured to come into contact with an inner circumferential surface of the external gear and move a meshing position of the internal gear and the external gear in a circumferential direction around the rotation axis, wherein
• the external gear includes a cylinder section, a diaphragm extending to a radial direction outer side of the cylinder section, and an annular boss section coupled to an outer circumferential end side of the diaphragm,
• the diaphragm includes a first coupling section coupled to an end portion of the cylinder section, a second coupling section coupled to an inner circumferential surface of the boss section, and a diaphragm main body coupled to the first coupling section and the second coupling section,
• a ratio of length from one opening end of the cylinder section to a surface of the diaphragm on an opposite side of the opening end in the rotation axis direction to length from the inner circumferential surface of the boss section coupled to the second coupling section to an 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 thickness of an inner circumferential end of the diaphragm and t(C) represents thickness of the outer circumferential end.