The present invention relates to the IL-12p40 subunit mutant gene which can produce IL-12(interleukin 12) of human and mouse origin with high activity, the expression vector including above mutant gene and the use of them to DNA vaccine adjuvant. Particularly, it relates to IL-12p40 mutant gene which inhibits the secretion of IL-12p40 but normally secretes active IL-12p70 by making mutation at Asn-222(human) or Asn-220(mouse) amino acid of IL-12p40, which acts as a competitive inhibitor of active form of IL-12, IL-12p70. Therefore, the IL-12p40 mutant gene of the present invention can be useful for DNA vaccination and gene therapy against various diseases, for example, AIDS, hepatitis C or hepatitis B, cancer, influenza, tuberculosis and malaria, which essentially require cellular immune responses for their therapy.