A marker for determining sensitivity of a patient to an anti-cancer agent, and novel cancer therapeutic means employing the marker, wherein the marker for determining sensitivity to an anti-cancer agent is a protein or a fragment, where in the protein or a fragment thereof exhibits a peak at m/z of 5,300 to 5,400, a peak at m/z of 6,130 to 6,230, a peak at m/z of 7,000 to 7,080 a peak at m/z of 7,840 to 7,920, a peak at m/z of 8,920 to 9,000, a peak at m/z of 12,440 to 12,560, a peak at m/z of 17,100 to 17,270, a peak at m/z of 18,290 to 18,470, a peak at m/z of 24,660 to 24,750, a peak at m/z of 35,980 to 36,290, a peak at m/z of 8,650 to 8,750, a peak at m/z of 9,100 to 9,200, a peak at m/z of 11,760 to 11,890, the peaks being determined by means of a mass spectrometer.