The present inventors successfully identified novel gatekeeper mutations for FGFR. Further, they discovered that mutant FGFR having the mutations demonstrate resistance to known FGFR inhibitors such as AZD4547, and at the same time demonstrate sensitivity to specific compounds. Mutant polypeptides having the mutations may be used as biomarkers in cancer treatment by FGFR inhibitors to prevent the development of side effects in therapy by conventional FGFR inhibitors, and to control the therapeutic mode for receiving the best therapeutic effect, thus making individualized treatment possible.