A rectangular nickel-metal hydride secondary cell permits improving the air-tightness and also permits preventing a metal case from being deformed in a step of folding an open end portion of the metal case. The rectangular nickel-metal hydride secondary cell, includes a rectangular cylindrical metal case having a bottom, an electrode group housed in the metal case, an alkali electrolyte contained in the metal case, a rectangular sealing plate mounted near the open end portion of the metal case. An insulating gasket is interposed in a compressed state between the inner wall near the open end of the metal case and the sealing plate. The metal case comprises a folded portion formed by inwardly folding the open end portion of the metal case and an inwardly projecting stepped portion formed along the inner surface of the metal case below the folded portion. The sealing plate is fixed to the metal case at the portion between the folded portion and the stepped portion of the metal case with the insulating gasket interposed therebetween, the folded portion at a corner portion of the open end portion of the metal case has an angle of 80.degree. to 100.degree.. Such a secondary cell can be formed by a process which includes charging-discharging cycles, an aging step, and a full charging step.