Disclosed are: a therapeutic agent, a kit and a treatment method for undifferentiated gastric cancer; and a pharmaceutical composition, a kit and a treatment method which are more effective on a living organism having at least one cell selected from the group consisting of a cell overexpressing FGFR2 and a cell expressing a mutant FGFR2. A combination of a FGFR2 inhibitor and a therapeutic substance for gastric cancer is more effective on undifferentiated gastric cancer. The combination of a FGFR2 inhibitor and a therapeutic substance for gastric cancer is more effective on a living organism having at least one cell selected from the group consisting of a cell overexpressing FGFR2 and a cell expressing a mutant FGFR2.