The present invention relates to a biomarker composition for diagnosing anticancer drug resistance of gastric cancer, including an FGFR2-ACSL5 fusion gene or a protein that the fusion gene encodes, a composition and a kit for diagnosing anticancer drug resistance of gastric cancer, including a preparation which detects an mRNA or protein of the fusion gene, and a method for diagnosing anticancer drug resistance of gastric cancer by using the biomarker. It is possible to diagnose in advance whether resistance to the drug will be exhibited during the treatment with the FGFR inhibitor by detecting an mRNA of the FGFR2-ACSL5 fusion gene according to the present invention or a protein which the gene encodes in a tissue derived from the patient with gastric cancer, so that the treatment efficiency can be enhanced by applying a more appropriate treatment to the patient.