In the present invention, as a result of detailed research into the expression, over-amplification, mutation and translocation, and the like of FGFR coding genes in a variety of cancer cells, a novel fusion polypeptide obtained by fusion of FGFR3 polypeptide with another polypeptide was identified and isolated in multiple urinary bladder cancer-derived cells and lung cancer cells. By using this fusion polypeptide as a biomarker in cancer therapy with an FGFR inhibitor, it is possible to preemptively prevent the manifestation of side effects in therapy with an FGFR inhibitor and control the therapeutic mode such that the best possible therapeutic results are obtained, and individualized medical treatment is possible.
