A preventative and/or therapeutic agent to treat and/or prevent organ failure due to mercury exposure, contains as the active ingredient a mutant tumor cytotoxic factor protein (TCF-II). The therapeutic agent comprises a mutant TCF-II protein according to SEQ ID NO. 2 in which amino acid residues Arg-Lys-Arg-Arg have been replaced by Ala-Ala-Ala-Ala, or in which amino acids Lys-Ile-Lys-Thr-Lys-Lys have been replaced by Ala-Ile-Ala-Thr-Ala-Ala (SEQ ID NO. 3).