The present invention relates to [7-[(7S)-7-amino-7-methyl-5-azaspiro[2.4]heptan-5-yl]-6-fluoro-1-[(1R,2S)-2-fluorocyclopropyl]-8-methoxy-1,4-dihydro-4-oxoquinoline-3-carboxylic acid hemihydrate. This hemihydrate is stable and easy to prepare, and has excellent properties for a pharmaceutical bulk powder.[7-[(7S)-7-胺基-7-甲基-5-氮雜螺[2.4]庚烷-5-基]-6-氟-1-[(1R,2S)-2-氟代環丙基]-8-甲氧基-1,4-二氫-4-氧代喹啉-3-羧酸]之1/2水合物係穩定且容易製備之化合物,其係具有作為藥品原料粉末優異之特性之化合物。