Isoleucine, offer of the anti- C type hepatitis virus active reinforcement medicine and the side effect reduction medicine of the IFN active substance which at least includes the divergence chain amino acid or that salt of 1 where it is chosen from the leucine, and the valine kinds.イソロイシン、ロイシン、及びバリンから選ばれる少なくとも1種の分岐鎖アミノ酸またはその塩を含むIFN作用物質の抗C型肝炎ウイルス活性増強剤及び副作用軽減剤の提供。