Provided is a novel cancer treatment method using an FGFR inhibitor that exhibits a very good antitumor effect and little adverse effect. Also provided are a combination preparation for treating malignant tumors that contains a compound represented by general formula (I) or a pharmaceutically acceptable salt thereof and one or a plurality of other compounds having an antitumor effect or pharmaceutically acceptable salts thereof； and a pharmaceutical composition that contains both agents. The following are also provided: an antitumor effect potentiator, an antitumor agent, a kit for treating malignant tumors, etc. that contain the compound represented by general formula (I) or a pharmaceutically acceptable salt thereof.
