A molecular marker for detecting a cancer stem cell in a cell mass which is a subject of interest, wherein the molecular marker can be detected in a cancer stem cell contained in the subject of interest but cannot be detected in a normal cell and a cancer cell that is different from a cancer stem cell; a method for determining the presence or absence of a cancer stem cell in a subject of interest by using the molecular marker as an measure; a kit for determining the presence or absence of a cancer stem cell, which comprises at least a reagent for detecting the molecular marker; a polypeptide encoded by the molecular marker; an antibody capable of recognizing an epitope of an expression product of a gene derived from the molecular marker; a nucleic acid capable of inhibiting the expression of the molecular marker; and a nucleic acid vaccine comprising a gene derived from the molecular marker.