In the cup-shaped externally toothed gear, the outside end face profile of the diaphragm is defined by a first concave circular arc having a first radius, a second concave circular arc that has a second radius and is smoothly connected to the first concave circular arc, an inclined straight line that is smoothly connected to the second concave circular arc and is inclined toward an inside straight line with respect to the center axis line, the inside straight line defining the outside profile of the diaphragm. The second radius is larger than the first radius, and the thickness of the diaphragm is gradually decreased from the side of the boss to the side of the cylindrical body. The stress concentration in the boss-side joint portion of the diaphragm can be relieved, whereby enhancing fatigue strength of the flexible externally toothed gear.