To provide an anti-cancer agent sensitivity determination marker, which marker can determine whether or not the patient has a therapeutic response to the anti-cancer agent, and novel cancer therapeutic means employing the marker. The anti-cancer agent sensitivity determination marker, the anti-cancer agent including oxaliplatin or a salt thereof and fluorouracil or a salt thereof, contains one or more substances selected from cysteine-glutathione, 2-aminoadipic acid and guanosine.