An object of the present invention is to provide a compound and pharmaceutical composition showing superior stability and/or solubility, etc. and having superior FLT3 inhibitory activity. The present invention provides a salt of (S,E)-N-(1-((5-(2-((4-cyanophenyl)amino)-4-(propylamino)pyrimidin-5-yl)pent-4-yn-1-yl)amino)-1-oxopropan-2-yl)-4-(dimethylamino)-N-methylbut-2-enamide, or a crystal thereof. Since the salt or a crystal thereof has superior FLT3 inhibitory activity and shows superior physicochemical properties as drugs, such as storage stability, solubility, and so forth, it is useful for treatments of diseases or conditions relating to FLT3. The present invention also provides a pharmaceutical composition and FLT3 inhibitor containing the salt or a crystal thereof.
