The present invention enables to realize a convenient determination of a therapeutic effect of an anticancer agent on a cancer. Specifically, the present invention provides a diagnostic marker for an effect of an anticancer agent on a cancer, comprising a substance having an affinity for a fragment of an extracellular domain of c-MET; a diagnostic reagent for an effect of an anticancer agent on a cancer, comprising a substance having an affinity for a fragment of an extracellular domain of c-MET and the fragment of the extracellular domain of c-MET; and a method of testing an effect of an anticancer agent on a cancer, comprising (a) measuring a concentration of a fragment of an extracellular domain of c-MET in a biological sample from a subject, and (b) comparing the measured concentration of the fragment of the extracellular domain of c-MET with an indicator which presents a relationship between a concentration of the fragment of the extracellular domain of c-MET and the effect of the anticancer agent on the cancer.