The purpose of the present invention is to provide: a detection agent for specifically detecting a cancer stem cell; a tumor antigen peptide specifically presented by cancer stem cells; a medicinal composition useful in preventing and/or treating cancer, said medicinal composition comprising the aforementioned tumor antigen peptide as an active ingredient; a method for screening the tumor antigen peptide; etc. To achieve the above-mentioned purpose, provided are: peptides represented by Y o -X o -Z o ; a polyepitope peptide consisting of a plurality of epitope peptides connected together, said polyepitope peptide containing at least one of the above-mentioned peptides as one of the epitope peptides; a polynucleotide encoding the aforementioned peptides and/or polyepitope peptide; a medicinal composition comprising the same as an active ingredient; a prophylactic and/or therapeutic agent for cancer characterized by inducing CTL; etc.