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)pyridine-3-yl]-5-ethyl-7- methoxy-4H-benz[d][1,3]oxazin-4-one maleate.