An object of the present invention is to elucidate a molecular mechanism of ligand-independent activation of EphA2 in cancer cells, make EphA2 a more useful target in the treatment of cancer, and provide a cancer testing method and the like using the mechanism. The present invention provides, for example, a cancer testing method including a step of measuring an amount of an EphA2 protein fragment having a molecular weight of from 30 kDa to 80 kDa in a sample derived from a subject.