Gear device and robot

A gear device comprising:

• an internal gear;
• an external gear having flexibility configured to partially mesh with the internal gear and relatively rotate around a rotation axis with respect to the internal gear;
• and
• a wave generator provided on an inner side of the external gear and configured to move a meshing position of the internal gear and the external gear in a circumferential direction around the rotation axis, wherein
• a main material of the internal gear includes graphite particles,
• a tooth surface of an internal tooth of the internal gear has a convex pattern including a first convex part and a second convex part extending in a first direction having a component along the rotation axis and arranged side by side in a second direction crossing the first direction, and
• 10≤D≤40 and S−D≤20, wherein D [μm] is an average particle diameter of the graphite particles and S [μm] is a separation distance between the first convex part and the second convex part in the second direction.