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 protruding from at least one side of outside peripheral portions of the electrode assembly ; and a battery casing including a first casing and a second casing provided with a first receiving part and a second receiving part , respectively, the first and second receiving parts respectively accommodating the electrode assembly and having different sizes or shapes, the method being characterized by forming electrode lead grooves 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 being recessed downwardly such that the electrode leads are seated therein, and a total depth (D3) obtained by adding a depth of said electrode lead grooves formed at the first casing and a depth of said electrode lead grooves formed at the second casing is smaller than a thickness of the electrode leads , the total depth (D3) being about 50% with respect to a thickness (T1) of the electrode leads , and the depths of the electrode lead grooves are equal to each other, and in a state in which the electrode leads seated in said electrode lead grooves 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 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 protruding from at least one side of outside peripheral portions of the electrode assembly ; and a battery casing including a first casing and a second casing provided with a first receiving part and a second receiving part , respectively, the first and second receiving parts respectively accommodating the electrode assembly and having different sizes or shapes, the method being characterized by forming electrode lead grooves 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 being recessed downwardly such that the electrode leads are seated therein, and a total depth (D3) obtained by adding a depth of said electrode lead grooves formed at the first casing and a depth of said electrode lead grooves formed at the second casing is smaller than a thickness of the electrode leads , the total depth (D3) being about 50% with respect to a thickness (T1) of the electrode leads , and the depths of the electrode lead grooves are equal to each other, and in a state in which the electrode leads seated in said electrode lead grooves 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 by thermal fusion along the outside peripheral portions (201, 201'; 401, 401') of the first and second casings (210, 410, 510; 220, 420, 520). <EOS>