An inhibitor against TGF-α expression and an inhibitor against transformation of a hepatitis and/or cirrhosis cell which comprise as an active ingredient a polyprenyl compound such as a polyprenyl carboxylic acid (for example, 3,7,11,15-tetramethyl-2,4,6,10,14-hexadecapentaenoic acid). The inhibitor can be used as an inhibitor against onset, recurrence (second oncogenesis), and malignant alteration of carcinoma in a liver.