[Problem] An object of the present invention is to elucidate a novel genetic mutation which is a cause of cancer, and thereby, to provide a method for detecting the genetic mutation, or a protein which has the mutation, a method for detecting the presence of cancer in a subject, a method for diagnosing cancer in a subject, and a primer set, a probe and a detecting kit for the methods.[Means for Solution] A method for detecting a fibroblast growth factor receptor 4 (FGFR4) mutant in a subject, which comprises a step of detecting the presence of the mutation of glycine at position 183 in a FGFR4 tyrosine kinase domain to cysteine in a sample obtained from the subject.