This invention provides an inducer of apoptosis in cancer cells comprising a fragment of the REIC/Dkk-3 gene and a cancer therapeutic agent comprising the same. This invention also provides a polynucleotide fragment encoding the REIC/Dkk-3 protein (a) or (b), which encodes a polypeptide having apoptosis activity: (a) a polynucleotide encoding a polypeptide comprising an amino acid sequence of amino acid 1 to any of amino acids 39 to 78 of the amino acid sequence of the REIC/Dkk-3 protein as shown in SEQ ID NO: 2； or (b) a polynucleotide encoding a polypeptide comprising an amino acid sequence derived from the amino acid sequence of amino acid 1 to any of amino acids 39 to 78 of the amino acid sequence of the REIC/Dkk-3 protein as shown in SEQ ID NO: 2 by substitution, deletion, or addition of 1 or several amino acids and having apoptosis activity.
