To provide a manufacturing method of an inexpensive armature shaft that has less vibration when rotated and that is excellent in durability.

A shaft material for manufacturing the armature shaft 7 is arranged in a shaft-forming recessed portion of a forging die by which a cold forging operation is performed. A sliding die 35, which has a forming recessed portion 35b that forms a spherical shape corresponding to a curved surface projection 15, is pushed into the forming recessed portion along the direction of an axial line L2 of the shaft forming recessed portion. The curved surface projection 15, which has an outside circumferential surface 15a which is less than a hemisphere of the shaft and on which a center point 01 is set on the center axial line L1 of the armature shaft 7, is formed on the end surface 11d of the shaft body 11 so that the armature shaft 7 is manufactured.