The problem to be solved by the present invention is to provide a potent and highly selective novel FGFR inhibitor, and an antitumor agent having reduced side effects, such as increased blood phosphorus levels, while maintaining the antitumor effect of the FGFR inhibitor. The present invention provides an antitumor agent comprising a 3,5-disubstituted benzene alkynyl compound represented by Formula (I) or a salt thereof that is used so that the 3,5-disubstituted benzene alkynyl compound or a salt thereof is administered on an administration schedule of at least twice a week and a dosing interval of at least one day.
