Provided are a pharmaceutical composition for treating an FLT3 mutation-positive cancer; a mutant FLT3 inhibitor; and uses thereof. Disclosed are a pharmaceutical composition for treating an FLT3 mutation-positive cancer, containing a compound represented by General Formula [1] or a salt thereof as an active ingredient; a mutant FLT3 inhibitor; an anticancer agent; a method for predicting a therapeutic effect by administration of a pharmaceutical composition containing a compound represented by General Formula [1] or a salt thereof in a subject, including a step of detecting the presence or absence of an FLT3 mutation; a method for selecting a subject to whom a pharmaceutical composition containing a compound represented by General Formula [1] or a salt thereof is applied, including a step of detecting the presence or absence of an FLT3 mutation; and a method for determining whether or not a pharmaceutical composition containing a compound represented by General Formula [1] or a salt thereof is administered to a subject, including a step of detecting the presence or absence of an FLT3 mutation.