The present invention replaces an amino acid residue of a specific region of a parent antibody so as to improve the stability of the antibody, thereby improving druggability. A variant antibody of the present invention, compared to a parent antibody hz1E11, has significantly improved stability while having nearly the same productivity and efficacy. Therefore, the variant antibody of the present invention exhibits, in the development of HER2-specific antibodies, excellent characteristics such as reduction in production costs, inhibition of efficacy reduction and reduction of side effects.