A prophylactic or therapeutic agent for a viral disease, characterized by combining 22β-methoxyolean-12-ene-3β,24(4β)-diol with an interferon as active ingredients is disclosed. The prophylactic or therapeutic agent of the present invention exhibits a high therapeutic effect by administering 22β-methoxyolean-12-ene-3β,24(4β)-diol and interferon as a combination thereof.