Provided is a pharmaceutical composition for treating cancer and asthma.The object can be solved by a pharmaceutical composition comprising a siRNA molecule that directs cleavage of mRNA of mucin subtype 5 AC via RNA interference, or an antibody against mucin subtype 5AC as an active ingredient. According to the pharmaceutical composition of the present invention, diseases involving an overexpression of mucin subtype 5 AC can be treated efficiently. In particular, the pharmaceutical composition comprising a siRNA molecule of the present invention is effectively used in the treatment of a cancer, particularly pancreatic cancer. In addition, the siRNA or the antibody of the present invention can inhibit the expression or function of MUC5AC, thereby preventing or treating the symptoms of asthma effectively.