The purpose of the present invention is to provide a salt of 4-(2-fluoro-4-(3-(2-phenylacetyl)thioureido)phenoxy)-7-methoxy-N-methylquinoline-6-carboxamide, which is useful as an antitumor drug, and crystals of the salt, the salt and crystals being excellent in terms of solubility, stability, and oral absorbability and being capable of mass-production. The present invention relates to a mesylic acid salt of 4-(2-fluoro-4-(3-(2-phenylacetyl)thioureido)phenoxy)-7-methoxy-N-methylquinoline-6-carboxamide and to the mesylic acid salt including crystals which give an X-ray powder diffraction spectrum having characteristic peaks at given diffraction angles.
