An object of the present invention is to provide a polypeptide capable of strongly inducing and activating dendritic-cell-like cells, so as to treat or prevent cancer by immunotherapy, and DNA encoding the polypeptide. The polypeptide is the following polypeptide (a) or (b) consisting of a partial region of the REIC/Dkk-3 protein: (a) a polypeptide consisting of a partial region of the REIC/Dkk-3 protein, which consists of an amino acid sequence selected from the group consisting of the amino acid sequences shown in SEQ ID NO: 3, SEQ ID NO: 5, SEQ ID NO: 7, and SEQ ID NO: 9 and the amino acid sequence of a polypeptide that contains the partial region consisting of Gly at position 205 to Phe at position 288 of the amino acid sequence shown in SEQ ID NO: 2 and consists of a fragment of the partial region consisting of Ser at position 135 to Ile at position 350 of the amino acid sequence shown in SEQ ID NO: 2 and (b) a polypeptide having activity of inducing dendritic-cell-like cell differentiation from monocytes, which is a polypeptide protein consisting of an amino acid sequence that has a substitution, a deletion, or an addition of 1 or several amino acids with respect to an amino acid sequence selected from the group consisting of the amino acid sequences shown in SEQ ID NO: 3, SEQ ID NO: 5, SEQ ID NO: 7, and SEQ ID NO: 9, and the amino acid sequence of a polypeptide that contains the partial region consisting of Gly at position 205 to Phe at position 288 of the amino acid sequence shown in SEQ ID NO: 2, and consists of a fragment of the partial region consisting of Ser at position 135 to Ile at position 350 of the amino acid sequence shown in SEQ ID NO: 2.
