The method of manufacturing a rotor core of an electric rotating machine includes a preliminary shaping step of forming a boss section having a cylindrical portion extending in an axial direction, a disk section extending from one end portion of the boss section in a radical direction and preliminary shape portions of claw sections extending from a periphery of the disk section in the axial direction, a boring step of forming an axial hole extending through the boss section in the axial direction, an ironing step of forming the claw sections, and a sizing step to achieve a necessary thickness and flatness of the boss section. The ironing step is performed with a mandrel having an interference with the axial hole smaller than a boring punch being inserted into the axial hole after completion of the boring step.