Disclosed are: a polypeptide which can strongly induce or activate a dendric-cell-like cell and can treat or prevent cancer through an immunotherapy using the polypeptide and DNA which encodes the polypeptide. Specifically disclosed is a polypeptide which comprises a partial region of REIC/Dkk-3 protein, and which is selected from: (a) a polypeptide which comprises a partial region of REIC/Dkk-3 protein, and which comprises an amino acid sequence that is selected from the group consisting of the amino acid sequences represented by SEQ ID NO:3, SEQ ID NO:5, SEQ ID NO:7 and SEQ ID NO:9 and an amino acid sequence for a polypeptide that contains a partial region lying between Gly at position-205 to Phe at position-288 in the amino acid sequence represented by SEQ ID NO:2 and that comprises a fragment of a partial region lying between Ser at position-135 to Ile at position-350 in the amino acid sequence represented by SEQ ID NO:2 and (b) a polypeptide which is a polypeptide protein comprising an amino acid sequence produced by substituting, deleting or adding one or several amino acid residues in an amino acid sequence that is selected from the group consisting of the amino acid sequences represented by SEQ ID NO:3, SEQ ID NO:5, SEQ ID NO:7 and SEQ ID NO:9 and an amino acid sequence for a polypeptide that contains a partial region lying between Gly at position-205 to Phe at position-288 in the amino acid sequence represented by SEQ ID NO:2 and that comprises a fragment of a partial region lying between Ser at position-135 to Ile at position-350 in the amino acid sequence represented by SEQ ID NO:2, and which has an activity of inducing the differentiation of a monocyte into a dendric-cell-like cell.Linvention concerne un polypeptide qui peut induire ou activer fortement une cellule de type cellule dendritique et qui peut traiter ou prévenir un cancer par immunothérapie au moyen du polypeptide, et un ADN qui code le polypeptide. Elle concerne spécifiquement un polypeptide
