A stem cell removing method that certainly-removes an undifferentiated stem cell is provided. For this object, a cell group including a stem cell and a somatic cell performed differentiation induction is cultivated in culture medium composition including photosensitizer. Light of a specific wavelength is irradiated with the cell group, and the stem cell is removed, selectively. The stem cell is a pluripotent stem cell or a somatic stem cell. The pluripotent stem cell includes either an ES cell (Embryonic Stem Cell) or an iPS cell (induced Pluripotent Stem Cell). Also, somatic stem cell includes any one of a germ stem cell, a productive cell, a pluripotent stem cell and a stem cell having unipotency.