At the time of manufacturing both a laminated rotor core 13 and a laminated stator core 14 by punching out a plurality of iron core pieces 11 and 12 from a same strip material 10 and laminating the punched out iron core pieces 11 and 12, respectively, a plurality of kinds of laminated rotor cores 13 and 13a are manufactured by further punching out a plurality of iron core pieces 11a from the strip material 10 and laminating the punched out iron core pieces 11a. Here, the plurality of kinds of the laminated rotor cores 13 and 13a can be used for different electric motors.