Method for producing outer shell

A method for producing an outer shell, comprising steps of:

• a) stacking a second plate on a first plate, the second plate having an accommodation space therein that is recessed toward the first plate;
• b) disposing a filling material in the accommodation space of the second plate;
• c) after step b), stacking a third plate on the second plate so as to cover the filling material and to form a composite structure;
• and
• d) after step c), stamping the composite structure such that a peripheral portion of the composite structure is bent, thereby forming the outer shell,
• wherein, in step b), a weight of the filling material is lower than a weight of the first plate, a weight of the second plate, and a weight of the third plate, or rigidity of the filling material is higher than rigidity of the first plate, rigidity of the second plate, and rigidity of the third plate.