A method of manufacturing a battery cell, wherein the battery cell comprises an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, with electrode tabs (131, 132) protruding from at least one side of outside peripheral portions of the electrode assembly; and a battery casing (200, 400, 500) including a first casing (210, 410, 510) and a second casing (220, 420, 520) provided with a first receiving part and a second receiving part, respectively, the first and second receiving parts (211, 221) respectively accommodating the electrode assembly and having different sizes or shapes, the method being characterized by forming electrode lead grooves (212, 222) at a portion of outside peripheral portions (201, 201'; 401, 401') of each of the first and second casings (210, 410, 510; 220, 420, 520), respectively, which correspond to one another, where electrode leads protrude, said electrode lead grooves (212, 222) being recessed downwardly such that the electrode leads are seated therein, and a total depth obtained by adding a depth of said electrode lead grooves formed at the first casing (210, 410, 510) and a depth of said electrode lead grooves formed at the second casing (220, 420, 520) is smaller than a thickness of the electrode leads, the total depth being about 50% with respect to a thickness of the electrode leads, and the depths of the electrode lead grooves (212, 22) are equal to each other, and in a state in which the electrode leads seated in said electrode lead grooves (212, 222) and connected to the electrode tabs protrude from the outside peripheral portions (201, 201'; 401, 401') of the first and second casings (210, 410, 510; 220, 420, 520), sealing the battery casing (200, 400, 500) by thermal fusion along the outside peripheral portions (201, 201'; 401, 401') of the first and second casings (210, 410, 510; 220, 420, 520). wherein the battery cell comprises an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, with electrode tabs (131, 132) protruding from at least one side of outside peripheral portions of the electrode assembly; and a battery casing (200, 400, 500) including a first casing (210, 410, 510) and a second casing (220, 420, 520) provided with a first receiving part and a second receiving part, respectively, the first and second receiving parts (211, 221) respectively accommodating the electrode assembly and having different sizes or shapes, the method being characterized by forming electrode lead grooves (212, 222) at a portion of outside peripheral portions (201, 201'; 401, 401') of each of the first and second casings (210, 410, 510; 220, 420, 520), respectively, which correspond to one another, where electrode leads protrude, said electrode lead grooves (212, 222) being recessed downwardly such that the electrode leads are seated therein, and a total depth obtained by adding a depth of said electrode lead grooves formed at the first casing (210, 410, 510) and a depth of said electrode lead grooves formed at the second casing (220, 420, 520) is smaller than a thickness of the electrode leads, the total depth being about 50% with respect to a thickness of the electrode leads, and the depths of the electrode lead grooves (212, 22) are equal to each other, and in a state in which the electrode leads seated in said electrode lead grooves (212, 222) and connected to the electrode tabs protrude from the outside peripheral portions (201, 201'; 401, 401') of the first and second casings (210, 410, 510; 220, 420, 520), sealing the battery casing (200, 400, 500) by thermal fusion along the outside peripheral portions (201, 201'; 401, 401') of the first and second casings (210, 410, 510; 220, 420, 520).