An internal planetary gear mechanism reduces a load applied to tooth surfaces of an external gear and an internal gear, a sliding portion such as a bearing and the like. By setting the amount of eccentricity α of an eccentric part to be larger than a theoretical value, a distance between the center of the external gear and the center of the internal gear is increased to allow the reduction of a loss in the bearing and on the tooth surfaces of the gears, and a mesh zone between the internal gear and the external gear can be reduced to decrease a part at a large pressure angle, thereby reducing a loss generated by a large pressure angle. As a result of reduction in the mesh zone between the external gear and the internal gear, rolling contact of the gears is maintained.