A prophylactic or therapeutic agent for a viral disease, characterized by combining 22²-methoxyolean-12-ene-3²,24(4²)-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 22²-methoxyolean-12-ene-3²,24(4²)-diol and interferon as a combination thereof.