To provide a marker for determining sensitivity of a patient to an anti-cancer agent, and novel cancer therapeutic means employing the marker.The marker for determining sensitivity to an anti-cancer agent is formed of one or more substances selected from the group consisting of a substance or a fragment thereof detected as an anion at m/z of 149.05 to 149.06, a substance or a fragment thereof detected as an anion at m/z of 152.99 to 153.00, a substance or a fragment thereof detected as a cation at m/z of 724.34 to 724.35, the peaks being determined by means of a mass spectrometer, glycerol 3-phosphate, dihydrobiopterin, GABA, lactic acid, asparagine, aspartic acid, 2-methylbutyroylcarnitine, 1-methyladenosine, and glutathione, and a substance involved in a metabolic pathway of any of these substances.