Provided are: a marker for determining the sensitivity to an anti cancer agent which can determine the therapeutic reactivity of individual patients and a novel cancer therapy means which utilizes the marker. A marker for determining the sensitivity to an anti cancer agent which comprises at least one gene selected from ALAD gene C20orf43 gene CABLES1 gene CDC14B gene GDA gene HOXB6 gene RPL7AP27 gene TMEM18 gene and UGT2B10 gene and contains oxaliplatin or a salt thereof fluorouracil or a salt thereof and levofolinate or a salt thereof.