An externally toothed gear configured to be used for a strain wave gearing, in which the externally toothed gear can be ellipsoidally flexed by a wave generator to form a state of partial meshing with a rigid internally toothed gear, the externally toothed gear comprising: a cylindrical barrel provided with one end being an open end; external teeth formed on a portion of an outer circumferential surface of the barrel on a side of the open end thereof; a diaphragm which is continuous with another end of the barrel and spreads outward in a radial direction of the barrel; and an annular boss formed continuously with an outer circumferential end of the diaphragm, wherein the diaphragm comprises: a diaphragm body portion extending in the radial direction; a curved portion which is continuous with an inner circumferential end of the diaphragm body portion, curves toward the barrel along a center axis from a direction inward in the radial direction, and is connected to the barrel; and a boss base portion which extends outward in the radial direction from the outer circumferential end of the diaphragm body portion, and is connected to an inner circumferential surface of the boss; and wherein a thickness of the diaphragm gradually decreases from a position of the boss base portion connected to the inner circumferential surface of the boss to a middle part between the outer circumferential end and the inner circumferential end in the diaphragm body portion, and the thickness of the diaphragm gradually increases from the middle part to the inner circumferential end of the diaphragm body portion; characterized in that thicknesses of the inner circumferential end, the middle part, and the outer circumferential end of the diaphragm, and a thickness of a barrel-side end part of the curved portion connected to the barrel satisfy t A > t C > t D > t B, where t is the thickness of the inner circumferential end, t is the thickness of the middle part, t is the thickness of the outer circumferential end, and t is the thickness of the barrel-side end part. a cylindrical barrel provided with one end being an open end; external teeth formed on a portion of an outer circumferential surface of the barrel on a side of the open end thereof; a diaphragm which is continuous with another end of the barrel and spreads outward in a radial direction of the barrel; and an annular boss formed continuously with an outer circumferential end of the diaphragm, wherein the diaphragm comprises: a diaphragm body portion extending in the radial direction; a curved portion which is continuous with an inner circumferential end of the diaphragm body portion, curves toward the barrel along a center axis from a direction inward in the radial direction, and is connected to the barrel; and a boss base portion which extends outward in the radial direction from the outer circumferential end of the diaphragm body portion, and is connected to an inner circumferential surface of the boss; and a diaphragm body portion extending in the radial direction; a curved portion which is continuous with an inner circumferential end of the diaphragm body portion, curves toward the barrel along a center axis from a direction inward in the radial direction, and is connected to the barrel; and a boss base portion which extends outward in the radial direction from the outer circumferential end of the diaphragm body portion, and is connected to an inner circumferential surface of the boss; and wherein a thickness of the diaphragm gradually decreases from a position of the boss base portion connected to the inner circumferential surface of the boss to a middle part between the outer circumferential end and the inner circumferential end in the diaphragm body portion, and the thickness of the diaphragm gradually increases from the middle part to the inner circumferential end of the diaphragm body portion; characterized in that thicknesses of the inner circumferential end, the middle part, and the outer circumferential end of the diaphragm, and a thickness of a barrel-side end part of the curved portion connected to the barrel satisfy t A > t C > t D > t B, where t is the thickness of the inner circumferential end, t is the thickness of the middle part, t is the thickness of the outer circumferential end, and t is the thickness of the barrel-side end part.