The .alpha. antigen-encoding gene and the .alpha. antigen protein suppress the production of interleukin-4 etc., improve the Th2 type cytokine-dominant state, and furthermore inhibit various conditions of allergic diseases such as IgE production, histamine release and eosinophil infiltration, and therefore they are very effective for the prevention or treatment of atopic diseases such as atopic dermatitis, asthma, allergic rhinitis, and allergic conjunctivitis, and more broadly allergic diseases.