To melt and diffuse metallic foreign bodies immixed in electrodes of a nonaqueous electrolyte secondary battery before initial charging, electrodes wound with a separator between a cathode plate and an anode plate are placed in a battery case and the battery case is filled with an electrolyte. After the case has been filled, the electrolyte is allowed to permeate into the electrodes. Then, the electrolyte-filled battery is placed in a processing device, and fixed with a surface pressure between at least 0.1 MPa and 5.0 MPa. Thereafter, the cathode potential is adjusted and held for a period of one hour and 35 hours while the battery remains fixed, after which the pre-initial charging process is terminated.