NOVELTY - The method involves mounting a sealing cover (3) in an opening portion of a square cylindrical portion (20). A gap of a former (401) is formed in the height direction in the upper outer peripheral surface from which the square cylindrical portion is contacted opposingly. A former (402) is stuffed into former (401) from the upper internal peripheral surface of square cylindrical portion by a protrusion portion. Protrusion portion is trimmed to form a level difference portion in the opening portion of square cylindrical portion, which is protruded outwardly.    USE - Manufacturing method of armored can used for manufacturing square secondary battery (claimed) installed in electric vehicle for photovoltaic power generation.    ADVANTAGE - The former is stuffed into another former from the upper internal peripheral surface of square cylindrical portion by the protrusion portion which is trimmed to form the level difference portion in the opening portion of square cylindrical portion, hence the thickness of the square cylindrical portion can be maintained reliably by the level difference portion. The formation of crack and fracture can be prevented effectively. The level difference portion is protruded outwardly, hence the reduction in the accommodation space of the armored can be prevented. The length of the height direction of the protrusion portion can be controlled efficiently. The molding of level-difference portion can be performed easily. The positioning shape of the sealing cover can be performed exactly. The stable dimension can be attained. The accommodation property can be reduced by making the formation location of the protrusion portion into the short-side region. The deformation of external shape can be decreased. The laser welding can be performed easily from upper direction.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for manufacturing method of secondary battery.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the armored can.    Sealing cover (3)    Square cylindrical portion (20)    Metal molding object (200)    Rectangular square cylindrical portion (202)    Formers (401,402)