Disclosed is a benz[d][1,3]oxazine derivative having excellent stability when used as an active ingredient for a medicinal agent, and also having excellent solubility and crystallinity. Specifically disclosed is 2-[2-((S)-3-dimethylaminopyrrolidin-1-yl)pyridin-3-yl]-5-ethyl-7-methoxy-4H-benz [d][1,3]oxazin-4-one maleate.