Abstract: The present invention aims to elucidate a polynucleotide as a novel responsible gene for cancer and aims to thus provide a method for detecting the polynucleotide and a polypeptide encoded by the polynucleotide and a detection kit, a probe set, and a primer set for the detection. The present invention also aims to provide a pharmaceutical composition for treating cancer. The method detects a fusion gene composed of a portion of an FGFR3 gene and a portion of a TACC3 gene or a fusion protein encoded by the fusion gene. The primer set, the probe set, or the detection kit comprises a sense primer and a probe set designed from the portion encoding FGFR3 and an antisense primer and a probe set designed from the portion encoding TACC3. Since an inhibitor of the polypeptide exhibits antitumor effect, a pharmaceutical composition for treating cancer which is positive for either the fusion gene or the polypeptide is provided.