The purpose of the present invention is to elucidate a polynucleotide which is a new cancer-causing gene, and to thereby provide: a method for detecting the polynucleotide or a polypeptide encoded by the polynucleotide a kit for detection a probe set and a primer set. Another purpose is to provide a pharmaceutical composition for treating cancer. This detection method detects a fusion gene that comprises a part of the FGFR3 gene and a part of the TACC3 gene, or a fusion protein encoded by the fusion gene. Said primer set, probe set, and kit for detection each contains a sense primer and a probe set which are designed from the part that encodes FGFR3 and an antisense primer and a probe set which are designed from the part that encodes TACC3. A cancer-treating pharmaceutical composition that is positive to said fusion gene or positive to said polypeptide is provided, because an inhibitor of said polypeptide exhibits an antitumor effect.
