Flexible external tooth gear, fluctuation speed reducer and robot

The flexible external gear has a cylindrical body portion and a diaphragm portion. The torso portion extends in a direction including a component of the central axis direction. The diaphragm portion extends from an axial end portion of the body portion in a direction including a radial component. The torso portion includes a first torso portion and a second torso portion. The first body portion is disposed on one side of the body portion in the axial direction and has flexibility. The second body portion is disposed closer to the other side in the axial direction than the first body portion. The second torso portion has a plurality of external teeth. Each of the plurality of outer teeth protrudes radially outward and is arranged in the circumferential direction. The maximum value of the thickness of the diaphragm part is less than twice the length of the outer teeth from the radial outer end to the radial inner side surface of the second body part, and the minimum value of the thickness of the first body part