An object of the present invention is to reveal a role of a stem cell control factor including Matrin-3 in cancer exhibiting neural differentiation including small cell lung cancer, and develop a medicine useful for treating cancer and the like based on the role. Among cancers having the cancer stem cell controlling mechanism, in cancer exhibiting the differentiation potency, the peculiar balance where both of expression of a stem cell maintenance factor and expression of a differentiation promoting factor are higher as compared with normal cells is retained. Additionally, in undifferentiated cancer, the peculiar balance where expression of a stem cell maintenance factor is higher as compared with normal cells is retained. In cancer in which the cancer stem cell controlling mechanism exists, these peculiar balances of a stem cell control factor different from the balance of normal cells exist. In the medicine for treating cancer and the method for treating cancer of the present invention, proliferativeness of cancer is suppressed by breaking down this balance of a stem cell control factor.