A wave gear device includes a circular spline that has an annular shape and rigidity; a flex spline that has an annular shape and flexibility, and is disposed in the circular spline; and a wave generator that is disposed in the flex spline, is configured to cause the flex spline to be distorted in a radial direction and is configured to partially mesh with the circular spline, and is configured to move a meshing position between the circular spline and the flex spline in a circumferential direction. The ratio of the Vickers hardness on an inner circumferential surface of the flex spline to the Vickers hardness on an outer circumferential surface of the wave generator is 1.2 or more and 1.7 or less.