The present invention relates to a method for manufacturing a battery cell, which is structured so that an electrode assembly having a structure in which a separation membrane is interposed between an anode and a cathode is sealed inside of a rectangular battery case. The present invention provides a method for manufacturing a rectangular battery cell, comprising the steps of: (a) manufacturing a rectangular main body of which the upper end and the lower end are open, by bending and then welding metal plates having a specific thickness; (b) manufacturing an upper end and a lower end sealing member having a shape that corresponds to the upper and the lower end of the rectangular main body; (c) bringing into close contact and then welding the lower end of the rectangular main body and the lower end sealing member; (d) inserting the electrode assembly into the battery case having the sealed lower end, which is manufactured in step (c); and (e) bringing into close contact and then welding the upper end of the battery case manufactured in step (d) and the upper end sealing member.