An apparatus for manufacturing an electric motor having a laminated core structure formed by a plurality of core plates made of rolled material of electromagnetic sheet steel. The core plates are manufactured by punching rolled electromagnetic sheet steel in a die set. The rolled electromagnetic sheet steel is displaced angularly relative to the die set. The core plates have rolling directions of the electromagnetic sheet steel angularly displaced by a machine angle determined in accordance with the number of slots or poles provided in the laminated core structure so that a phase difference of cogging torques produced by magnetic anisotropy of the core plates is substantially 180.degree..