Abstract: Provided is a novel marker for determining anti-cancer agent sensitivity. The present invention provides a marker for determining sensitivity to an anti-cancer agent, the anti-cancer agent including irinotecan or SN-38 or a salt thereof, fluorouracil or a salt thereof, and levofolinate or a salt thereof, the marker comprising one or more molecules selected from the group consisting of 5A4CR, ALA, ASP, CYS, CSSG, GLC3P, HIS, ILE, LEU, LYS, METSF, N6TLY, N6ALY, OCTA, SER, TUCA, THR, TRP, TYR and VAL. The present invention also provides a marker for determining sensitivity to an anti-cancer agent, the anti-cancer agent including irinotecan or SN-38 or a salt thereof, fluorouracil or a salt thereof, and levofolinate or a salt thereof, the marker comprising one or more molecules selected from the group consisting of 3IND, 4OVAL, 5A4CR, ALA, BENZA, CREAT, CSSG, DECNA, GABB, GLC3P, HYPTA, LYS, METSF, N8ASR, QUINA, SARCO, TMNO and VAL.