Differential side gear and a method of manufacturing the same

And differential side gear in which the plug member is able to achieve the rigidity of and bevel gears can be reliably prevented reliably prevent can and the hydraulic oil leaking from falling out of the bevel gears of the differential side gear, to provide a method of manufacturing the same is there. A differential side gear of an automotive differential gear device includes a bevel gear portion, a cylindrical shaft portion integrally formed with the bevel gear portion includes a shaft insertion hole comprising a spline shaft coupling hole, the shaft insertion hole splines comprising a plug fitting hole formed on the inner wall portion of the bevel gears so as to communicate with the axial coupling hole, provided a metal plug member that is press-fitted into the plug fitting hole, said outer peripheral portion of the plug member plug an annular weld was welded in a liquid-tight manner to the inner circumferential wall of the fitting hole is provided, to form an annular weld by the capacitor resistance welding while press-fitting the plug member to plug fitting hole.